lib/bitmap: Faster bitmap functions.

Replace bitwise loops with a single operation, inline all bitmap
functions.  Inlining allows the compiler to remove unnecessary code
due to some parameters being compile-time constants.

Before:

$ tests/ovstest test-bitmap benchmark 1000000
bitmap equal:    341 ms
bitmap scan:   8089 ms

After:

$ tests/ovstest test-bitmap benchmark 1000000
bitmap equal:    152 ms
bitmap scan:    146 ms

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Co-authored-by: Kmindg <kmindg@gmail.com>
Acked-by: Ben Pfaff <blp@nicira.com>