Introduce git__reallocarray that checks the product of the number
of elements and element size for overflow before allocation.  Also
introduce git__mallocarray that behaves like calloc, but without the
`c`.  (It does not zero memory, for those truly worried about every
cycle.)

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

When turning UTF-8 paths into UCS-2 paths for Windows, always use
the \\?\-prefixed paths.  Because this bypasses the system's
path canonicalization, handle the canonicalization functions ourselves.

We must:
 1. always use a backslash as a directory separator
 2. only use a single backslash between directories
 3. not rely on the system to translate "." and ".." in paths
 4. remove trailing backslashes, except at the drive root (C:\)