Implement STDOUT, STDERR and STDIN entirely in Inko

[yorickpeterse]

Description

These types currently make use of the Rust runtime library, and wrap the Rust types. There's however no need for this:

• We don't care about Windows for the time being, so we can just open /dev/stdout / etc
• Since we now have conditional compilation, handling Windows in the future shouldn't be difficult
• Rust imposes a lock around STDOUT, STDERR and STDIN, as these can be used in global ways (e.g. through the println!() macro). This isn't needed for Inko though since we require one to create an explicit instance, and there's no way to stick these types in a global

Implementation wise I'm leaning towards not buffering STDOUT by default. With 988b72191f4aa66a3ac4f95dcf07b88634bad7de it's possible to perform buffered writes, meaning you can opt-in to buffered writes if desired, while opting out would require a separate type (e.g. STDOUT and UnbufferedSTDOUT/UnbufferedStdout?).

As part of this we should also rename these types as follows:

• STDOUT -> Stdout
• STDERR -> Stderr
• STDIN -> Stdin

Blocked by

• https://github.com/inko-lang/inko/issues/633

Related work

• https://endler.dev/2017/yes/
• https://eklitzke.org/stdout-buffering

[yorickpeterse]

To add another reason for not buffering STDOUT: for certain scenarios you might want to automatically disable buffering, such as when running the program in a pipeline. For this we'd need https://github.com/inko-lang/inko/issues/634, which in turn requires more work.

[yorickpeterse]

A brief overview of what some languages do:

Language	STDOUT
C#	buffered
Crystal	buffered, though it has a helper method for automatically flushing
D	buffered
Go	unbuffered?
Julia	line buffered?
Node.js	unsure, seems to be synchornized?
Python	buffered
Rust	line buffered
Swift	block buffered?

[yorickpeterse]

To add to the above: while the language may do one thing, the underlying platform may do something else. For example, libc still buffers STDOUT by default unless you explicitly disable this using setvbuf().

[yorickpeterse]

Another note: if we want to ensure that STDOUT and such always use the file descriptors 0, 1, and 2, we need to first take care of https://github.com/inko-lang/inko/issues/633, such that files in Inko are just wrappers around file descriptors instead of opaque Rust values.