Migrate to our own Signal type

This is part of #353, and rework of #365.

This PR removes the use of nix::signal::Signal in the yash_env::System API. The definition of Signal is dependent on the underlying system, so it should not be used outside RealSystem. Instead, new neutral items are added in the yash_env::signal module.

The new Number type replaces the most use of Signal. The RealSystem and VirtualSystem now have different Number spaces so that the VirtualSystem no longer has to depend on the RealSystem's signal number. The other uses of Signals are replaced by Names, which is more abstract than Numbers.

This PR also adds (partial) support for real-time signals.

To fully support real-time signals, we also need to eliminate the use of SigSet, which is not the scope of this pull request.

Defining new items
- [x] Define new signal items (Number and Name)
- [x] Implement From<Number> for ExitStatus
- [x] Implement System::signal_number_from_name and System::validate_signal_number
- [x] Implement SystemEx:signal_name_from_number
- [x] Implement SystemEx:signal_number_from_exit_status
- [x] Implement new job state reporting ~~which depends on System and will replace yash_env::job::fmt~~
Updating existing items
- [x] Use Number in virtual Process
- [x] Let ProcessResult contain Number and update the implementation of From<ProcessResult> for ExitStatus, etc.
  - This change requires first removing the old yash::job::fmt, which depends on the old ProcessResult definition.
- [x] Let yash_env::trap::Condition contain Number and update dependent implementations accordingly
- [x] Change the signature of System::sigaction
- [x] Change the signature of System::caught_signals
- [x] Change the signature of System::kill
- [x] Change the signature of SelectSystem::set_signal_handling
- [x] Change the signature of SharedSystem::wait_for_signals
- [x] Change the signature of SharedSystem::wait_for_signal
Updating built-ins
- [x] Update the fg built-in implementation
- [x] Update the kill built-in implementation
- [x] Update the set built-in test
- [x] Update the trap built-in implementation
- [x] Update the wait built-in implementation
- [x] Update the jobs built-in implementation
Removing old items
- [x] Remove the old yash::job::fmt items
- [x] Remove ProcessState::from_wait_status by inlining the function
- [x] Remove impl TryFrom<Signal> for signal::Number
- [x] Remove impl From<Signal> for ExitStatus
- [x] Remove the old signal items
Refactoring
- [x] Split yash_builtin::trap::Command::execute into smaller functions

Summary by CodeRabbit

New Features
- Added support for real-time signals in the kill built-in.
- Introduced new entities for improved signal handling and job control.
Improvements
- Enhanced signal parsing and handling logic for better maintainability and clarity.
- Updated signal-related functions to use more precise data types and error handling.
Bug Fixes
- Corrected various function signatures and return types to improve consistency and error reporting.
Documentation
- Updated documentation to reflect changes in signal handling and job control mechanisms.

Walkthrough

The changes in the yash-builtin and yash-env modules primarily focus on enhancing signal handling. This involves adding new signal-related functionalities, refactoring existing signal logic, and updating function signatures and structures to accommodate these improvements. Key updates include new methods for signal validation, mapping, and handling, along with modifications to job control, trap, kill, and wait mechanisms to use these new signal handling capabilities.

Changes

File(s)	Change Summary
`yash-builtin/CHANGELOG.md`	Summarized changes in version `0.2.0`, highlighting additions, modifications, and removals.
`yash-builtin/src/bg.rs`, `fg.rs`	Updated signal handling logic, replacing direct signal usage with signal names.
`yash-builtin/src/jobs.rs`	Modified job control mechanisms, renamed fields, and updated method signatures.
`yash-builtin/src/kill.rs`, `print.rs`, `send.rs`, `signal.rs`, `syntax.rs`	Refactored signal handling, parsing, and execution logic, introduced new structures and methods.
`yash-builtin/src/set.rs`	Updated signal references to use new modules and structures.
`yash-builtin/src/trap.rs`, `cond.rs`, `syntax.rs`	Enhanced trap handling with new condition specifications and error handling improvements.
`yash-builtin/src/wait/core.rs`	Adjusted error handling and signal references in wait mechanisms.
`yash-env/src/system.rs`	Introduced new methods for signal validation, mapping, and handling, modified existing methods.

Sequence Diagram(s)

sequenceDiagram
    participant User
    participant yash-builtin
    participant yash-env

    User->>yash-builtin: Send signal via kill command
    yash-builtin->>yash-env: Validate and parse signal
    yash-env-->>yash-builtin: Return signal number
    yash-builtin->>yash-env: Execute signal handling
    yash-env-->>yash-builtin: Signal action performed
    yash-builtin-->>User: Confirmation of signal execution

Poem

In the code where signals play,
A rabbit hops through night and day.
With traps and kills, it deftly weaves,
Ensuring all the bugs it leaves.
Signals parsed with utmost care,
In yash, a system rare.
Hooray for changes, hip and bright,
The rabbit's code, a shining light! 🌟🐇

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magicant / yash-rs