Micro-optimize PFR fields detection

Start upper bound fields search from 4 fields, to avoid slow startup on typical workloads
Inline the fields_count_binary_search_unbounded function to reduce template instantiations depth by 1
Renamed min to min_of_size_t to avoid weired syntax
Applied idea of better error reporting from #120
Do not start fields count computation if one of the static asserts failed. That speedups error reporting in edge cases
Use std::*_t versions of traits as they are faster in some implementations
Rewrite binary search to simplify it and to avoid degradation to linear search on types that have constructor from variadic pack
Remove default template parameters to simplify code

As a result, the whole test suite now runs 10%-25% faster on MSVC, ~20% faster on Clang, and 7%-20% faster on GCC.

boostorg / pfr