This patch includes an intrinsic implementation for these functions
when the compiler is GCC, Clang, or MSVC--or if the compiler claims
to be one of those.  Otherwise, a naive implementation is used.

Tests are included for all of these functions, which turned up a
possible problem: according to the gcc documentation, __builtin_ctz
and __builtin_clz give an undefined result when their inputs is
zero.  I was not able to persuade myself that we always called them
with a nonzero argument.