Optimize [nmd]alloc() fast paths.

Optimize [nmd]alloc() fast paths such that the (flags == 0) case is
streamlined, flags decoding only happens to the minimum degree
necessary, and no conditionals are repeated.
Move typedefs from jemalloc_protos.h.in to jemalloc_typedefs.h.in.

Move typedefs from jemalloc_protos.h.in to jemalloc_typedefs.h.in, so
that typedefs aren't redefined when compiling stress tests.
Add a simple timer implementation for use in benchmarking.
Add microbench tests.