Adds support for local context loads and stores. Also adds support for
creation of new block contexts (e.g., for let variables) and initializing
const / let variables with the hole appropriately.

Also adds some checks to ensure BytecodeArrayBuilder::context_count is set
appropriately and fixes tests to do so.

Adds the bytecode StaContextSlot.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1403943004

Cr-Commit-Position: refs/heads/master@{#31343}