This unifies the memory management of identifier strings passed between
the scanner, parser and module builder. The following scheme is used:
 - The scanner does not create copies of identifier strings itself, it
   exposes a reference to the current identifier. This reference becomes
   invalid as soon as the scanner advanced.
 - The parser preserves a single copy of each identifier that is stored
   in any data structure. That copy is allocated in the zone, lifetime
   is coupled to that of the zone.
 - The module builder can use all such identifiers by reference, as long
   as its lifetime is also coupled to the same zone.

Note that the module builder still creates redundant copies for some
identifiers (in order to maintain backwards compatibility with the old
AST-based parser). This can be fixed once the "old validator" has been
removed.

R=clemensh@chromium.org
BUG=v8:6127

Change-Id: I8611d162e87730045a6061d08c3fe841daae8a7d
Reviewed-on: https://chromium-review.googlesource.com/484439
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44836}