Add full constexpr support for small_vector and small_string

[cschreib]

This PR does the following:

• Remove un-initialised data in small_vector and small_string, which removes all chances of undefined behavior (UB). The potential UB was preventing these classes from being used in constexpr settings. Closes small_string cannot be used in constexpr expression if string isn't full #17.
• When exceptions are disabled, all REQUIRE*() and FAIL() macros will call std::terminate instead of return, to ensure no further code in the test case is executed.

[codecov-commenter]

Codecov Report

Merging #38 (eddc563) into main (4f69528) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #38   +/-   ##
=======================================
  Coverage   92.96%   92.96%           
=======================================
  Files           2        2           
  Lines         981      981           
=======================================
  Hits          912      912           
  Misses         69       69           

Impacted Files	Coverage Δ
include/snatch/snatch.hpp	96.11% <ø> (ø)
src/snatch.cpp	91.33% <ø> (ø)

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