deprectaed in c++17 out-of-line definition of constexpr static data member

[arturbac]

There are plenty of deprecated out of line static constexpr declarations
You can easy get rid of them with clang and

target_compile_options(glaze_glaze INTERFACE -Werror=deprecated-redundant-constexpr-static-def)

which will produce errors

error: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated

[stephenberry]

I fixed one location of this error in #1200, however most of these occur due to https://github.com/fastfloat/fast_float, and I have raised an issue there for them to be fixed.