Add CONSTEXPR_CHECK and CONSTEXPR_REQUIRE

[cschreib]

This PR introduces full support for compile-time testing. This includes compile-time evaluation and expression decomposition. For this purpose, two new sets of macros are included:

• CONSTEVAL_* performs a check purely at compile-time, reports failure to compile as constant expression at compile-time (duh), and failure of the test at run-time. No code (other than the potential failure reporting) is executed at run-time, therefore this will not generate coverage data.
• CONSTEXPR_* performs a check both at compile-time and run-time. Failure to compile as constant expression is still reported at compile-time, and failure of the test is still reported at run-time. But because the code is executed both at compile-time and at run-time, this will generate coverage data.

Although we already know at compile-time if a compile-time test passes or fail, the test failure is nonetheless only reported at run-time. This is done to follow the same reporting mechanism as the one set up for regular run-time tests (custom reporter, etc.).

To achieve this, I needed to implement constexpr integer and float serialization functions, since the C++20 STL does not provide anything usable at compile time (in C++23 we would be able to use std::to_chars, but only for integers and not for floats). Although integer serialization is pretty easy, float is not. The algorithm I used is based on fixed point arithmetic with 64 bit worth of digits in base 10. It is a simpler form of common state-of-the-art algorithms (e.g. Grisu). It is probably nowhere near as efficient, but on the plus side doesn't require storing large tables or complicated arbitrary precision types. To keep it simple, I only implemented exponential format with fixed precision (6 digits after the decimal point for float, and 15 for double). This achieves a perfect match to std::printf() for floats and 6 digits precision in 99.995% of the cases (when it fails, it's just the very last digit that is off by one). I don't have the stats for doubles.

I also needed to refactor the expression decomposer a little bit, to be constexpr-friendly. This had the nice side effect of improving compilation times slightly overall.

Closes #77.
Closes #60.

[codecov]

Codecov Report

Merging #76 (d6cb28d) into main (793baf8) will increase coverage by 0.04%.
The diff coverage is 94.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
+ Coverage   92.56%   92.61%   +0.04%     
==========================================
  Files           2        2              
  Lines        1049     1272     +223     
==========================================
+ Hits          971     1178     +207     
- Misses         78       94      +16     

Impacted Files	Coverage Δ
include/snitch/snitch.hpp	95.03% <94.38%> (-1.03%)	⬇️
src/snitch.cpp	90.67% <100.00%> (-0.29%)	⬇️

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