See https://github.com/boostorg/cmake/issues/45#issuecomment-1815570996.

headers are header-only libs, boost are compiled libs.

This guarantees floating-point -> text -> floating-point
roundtrip.

Those places, where the output precision was explicitly
set to 15-18 places, were replaced with max_digits10
precision.

This slightly simplifies code avoiding extra boolean variable.

These functions cannot return a dangling reference.

Remove unused version of findElementOrError with iterators
and error string.

Using in MPL shows no difference in runtime, one assembly
instruction is different.

Fixes a warning on newest gcc.