Allocate class instances statically

Issue

The classes that make up the AYAB firmware are currently dynamically allocated with new at the top level of main.cpp:

https://github.com/AllYarnsAreBeautiful/ayab-firmware/blob/e960ad4d9cd86daa4b3abbafad90de51ad102da5/src/ayab/main.cpp#L50-L56

Since these objects are allocated at the start of the program and never freed, there is no advantage to having them dynamically allocated. On the other hand, there are disadvantages:

dynamically allocated objects can use a few more bytes for the memory allocator's bookkeeping;
the memory used by these objects is not counted in the PlatformIO builder's statistics of RAM usage, that are printed after every build.

That last point in particular can hide the fact that the memory requirements for the firmware exceed the target microcontroller's RAM size (2048 bytes for the Atmega328p on an Arduino Uno).

Proposed solution

This PR replaces the use of new by the creation of statically allocated instances. The global pointers are then initialized to point to these instances.

Before this PR, here is what the output of the PlatformIO builder looks like:

RAM:   [======    ]  60.2% (used 1232 bytes from 2048 bytes)

Which looks reasonable. But after including the firmware's main objects in the statically allocated area as this PR does, here is the output:

RAM:   [========= ]  91.3% (used 1869 bytes from 2048 bytes)

Revealing that the memory actually available for the stack is much less than was suggested by the earlier output, causing issues like #190.

The memory saved here seems to fix #190 partially. At least, even with stack overflow detection enabled, the firmware can now survive reqInit and even knitting, from my quick test.

Note that this PR also deletes a series of uninitialized constant pointers that appear to have been remains from an earlier attempt at these global pointers.

Summary by CodeRabbit

New Features
- Improved instance management for key components by replacing dynamic memory allocation with stack-allocated instances, enhancing performance and reducing memory leak risks.
Refactor
- Streamlined the initialization process of singleton instances, resulting in clearer and more maintainable code.
Bug Fixes
- Addressed potential memory management issues by eliminating the use of new for singleton instances.

[!CAUTION]

Review failed

The pull request is closed.

Walkthrough

The recent changes in src/ayab/main.cpp involve a significant refactor of singleton instance management. Instead of using constexpr pointers to dynamically allocated instances, the code now utilizes stack-allocated instances, enhancing memory management safety and performance by eliminating potential memory leaks and overhead from heap allocations.

Changes

Files	Change Summary
`src/ayab/main.cpp`	Replaced `constexpr` pointers to singleton classes with direct stack-allocated instances.

Assessment against linked issues

Objective	Addressed	Explanation
Memory gets corrupted when processing serial messages (#190)	✅

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AllYarnsAreBeautiful / ayab-firmware