Avoid warnings from -fsanitize=integer in the hash functions.

Avoid warnings from -fsanitize=integer in the hash functions.

Hash functions rely heavily on unsigned integer overflow behavior, but
the sanitizer complains about them. Instead of disabling the sanitizer
(which might prevent us from getting warnings from real errors), silence
it by explicitly upcasting values to 64-bit integers before applying
operations, then explicitly downcasting to 32-bit again. The compiler
will optimize this out.