More code cleanups in stl/src

[StephanTLavavej]

• Remove unnecessary parentheses in _tolower.cpp, _toupper.cpp, xwctomb.cpp.
  • (Comparisons have well-known higher precedence than logical operators.)
• Use C++ casts, working towards src: Change C casts to C++ casts #184.
  • Use static_cast for arithmetic types.
  • Use reinterpret_cast for pointers.
  • Use static_cast from void*.
    • (This is providing type information after malloc etc., or restoring type information after going through a void* API. We've stylistically preferred static_cast instead of reinterpret_cast in these scenarios.)
  • static_cast<const char*> when memchr() returns const void*. We don't need to cast away const.
  • We don't need to cast wmemchr() at all; its return type is already correct.
  • Use reinterpret_cast for _Dval*/_Fval*/_Lval* (union type-punning for floating-point). Drop the reinterpret_cast<char*>, which I believe was intended to prevent aliasing optimizations. Remove unnecessary parentheses afterwards. Use const auto (our new convention after a cast).
  • Use const_cast, required by the mismatched type of endptr.
  • In _tolower.cpp, const_cast<short*> then implicitly convert to void*.
  • static_cast and const_cast to locale::facet* in locale.cpp, wlocale.cpp, xlocale.cpp.
    • (These files should probably be unified, and this repeated macro should probably be replaced by a function template, but I haven't attempted to perform such an invasive change here.)
• In xxxprec.h, don't macroize ldexpf/sqrtf; the UCRT provides them.
• Change 0 to nullptr.
• In primitives.h, change __alignof to alignof.
  • (I believe this was attempting to be usable by C translation units, but we compile everything as C++ now. I haven't attempted to change the usage of __max yet.)
• In filesys.cpp, change wchar_t(0) (which is technically a function-style cast) to L'\0'.
• In xstoul.cpp, remove preprocessor logic because unsigned long is always 32-bit on our platforms.
• In locale.cpp, wlocale.cpp, xlocale.cpp, use default template args and name typedefs consistently.
  • (I verified that the types being passed were the defaults.)
• Fix preprocessor comment in locale0.cpp.
• Adjust test strings in tr1.
  • (This avoids an astronomical inaccuracy and unnecessary trademark usage.)
  • (The important characteristics, of having an embedded newline and a space, are preserved.)
• Move a few definitions down to their initializations.
• Extract a few assignments before if-statements.
• In mutex.cpp, avoid an elaborated-type-specifier (presumably a relic of when this was a C translation unit).

[CaseyCarter]

Feel free to note in the commit log that any breakage from removing the double reinterpret_casts is my fault ;)