Rename tor_mem_is_zero to fast_mem_is_zero; use it consistently

By convention, tor_memeq() means "constant time" and fast_memeq() means "variable-time." But tor_mem_is_zero is variable-time, whereas its constant-time variant is safe_mem_is_zero.

I'm fine with the "safe_" name, but we should rename tor_mem_is_zero() to indicate that it is the fast one, not the safe one. We should also audit its users, particularly tor_digest{256,}_is_zero()