After moving to its own header, this CL cleans up some parts of the
interface. It fixes names and const-declarations of simple accessors,
and adds a named constructor to make it explicit that an EnumSet should
be constructed from an integral value.
Also refactor the use in cctest.h to have less statically declared
constants. Instead, just create the set of extensions in the individual
tests.

R=titzer@chromium.org

Bug: v8:8562
Change-Id: I6178d1aba25afa1d7f54c29ccf81505c165e7cd3
Reviewed-on: https://chromium-review.googlesource.com/c/1409366
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58862}