Use compile-time bounds checking when possible

[tcbrindle]

We know that the compiler, when optimising, can remove runtime bounds checks when it can prove that they will never fail.

If would be nice if we could have the converse as well: if the compiler can prove that a "runtime" bounds check is certain to fail, then that should be a compile error.

And that's what this PR does. It uses GCC's __builtin_constant_p to ask whether the index and size parameters are known constants, and if they are (and the bounds check fails) it tries to call the flux::static_bound_check_failed() function. This function is undefined, but more importantly it's marked with the [[gnu::error]] attribute, meaning that we'll get a compile error rather than a linker error, hopefully with a nice backtrace.

This works with GCC and Clang, at -O1 or above (presumably, when enough constant-folding happens in the front end). Otherwise, we'll get a runtime error as normal.

The extra checks can be disabled by defining FLUX_DISABLE_STATIC_BOUNDS_CHECKING before #include-ing Flux.

Thanks to @mattkretz for showing me this trick and suggesting I use it in Flux!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7b4cdf6) 97.70% compared to head (78cef5f) 97.71%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #137   +/-   ##
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  Coverage   97.70%   97.71%           
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  Files          67       67           
  Lines        2400     2402    +2     
=======================================
+ Hits         2345     2347    +2     
  Misses         55       55           

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