Adds support for creation of new local function contexts (or script context for
top-level code). As part of this, also adds support for context push/pop
operations using a ContextScope object in BytecodeGenerator. Adds the following
bytecodes:
 - PushContext
 - PopContext

Support for inner contexts and loading from / storing to context allocated
variables will come in a future CL.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1379793004

Cr-Commit-Position: refs/heads/master@{#31238}