This makes the debug-only scope-name actually debug-only-allocated, replaces num_vars_ usages by variables_.occupancy, and shuffles fields around in the scope class for better packing.

This reduces sizeof(i::Scope) from 360 to 328 bytes on x64.

BUG=v8:5209

Review-Url: https://codereview.chromium.org/2201763004
Cr-Commit-Position: refs/heads/master@{#38210}