<mutex>: change mutex to be an SRWLOCK

Description
std::mutex is currently powered by ConcRT to support Windows XP. As a result the size of mutex is too big to fit in a cache line.

When XP support can be dropped, the size can be reduced to sizeof(void*) by changing mutex to be an SRWLOCK, which is the highest performance lock provided by Windows.

Semi-duplicate description

• From #654 <mutex>: Implement mutex and recursive_mutex on SRW Lock. Support recursion only on recursive_mutex
  • correctness (constexpr construction)
  • no undesired strengthening (recursion on non-recursive deliberately not behaving correctly)
  • performance (no dynamic API binding => no control flow guard overhead; no recursion support overhead)

Additional context
See also: Developer Community DevCom-334696 and Microsoft-internal VSO-685103 / AB#685103.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.

Start with the <mutex> implementation and the vNext guidance in #169; the issue names no source file or test. Work is gated on the vNext branch, and done means using SRWLOCK while preserving constexpr construction, limiting recursion to recursive_mutex, and accepting the stated binary-compatibility break.