Better handling of internal failure modes and fix section bug

[cschreib]

This PR does the following:

• Add guidelines for contributions.
• Refactor snitch::small_function to stronger invariants (always contains a function to call). This is technically a breaking change (the class lost its empty() member, and is no longer default-constructible), but this should not affect anyone using snitch as a testing framework.
• Refactor the default reporter to use the standard reporter interface, rather than special treatment.
• Replace several calls to std::terminate to a customizable assertion handler (terminates by default, but can be made to throw instead for testing).
• Apply the Lakos Rule for noexcept to the whole code. This led to a minor but measurable performance degradation (on compilation time mostly).
• Add tests for various failure modes which used to unconditionally terminate.
• Fixes #82.
• Fixes #83.

[codecov[bot]]

Codecov Report

Merging #81 (2800a42) into main (d6cb28d) will increase coverage by 0.68%. The diff coverage is 92.61%.

Additional details and impacted files

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