First pass on reentrant C++-ish parser

We're flipping Flex/Bison in to reentrant mode, cutting down on global
variables, and exposing a more C++-like interface earlier. This is the first
phase. There's still a couple of weird hacks to deal with the previous code's
reckless disregard for scope sanity, but the outline of things to come is
starting to appear, and this CL really doesn't need to get any bigger.

Change-Id: Ife2d70db026d7ab1319bdf6c586959315666d0bb
Signed-off-by: Casey Dahlin <sadmac@google.com>