Add REOP_char8 that matches single bytes. Compresses bytecode for the
ASCII common case by 33% and reduces regexp_ascii benchmark running time
by 4%. The regexp_utf16 benchmark is unaffected.
The removal of the high-precision but non-standard clock source in
commit 5af98ca broke microbench because Date.now() is not granular
enough for the benchmark runner to make forward progress.
This commit adds a new method to the os module that returns time
with microsecond precision.
