C: Thread safety and improved MSVC build support

[rcombs]

This is derived from #72, but with the commits restructured and simplified, and some additional changes.

• Fix building on MSVC
  • Fix incorrect static array size
  • Remove implicit type conversions
  • Remove VLA usage
• Expose a thread-safe version of the main hash function

The original main hash function is now implemented in terms of the thread-safe version.

[mrousavy]

Does this come with a performance cost?

[rcombs]

A small one, but yes; you could alternately solve the same problem by making the array [9][9][3] at no cost.