[es6] Refactor FormalParameter

[es6] Refactor FormalParameter

Store arity in FormalParameters; store name (instead of var) and is_rest flag in individual parameters. Ensure that the arity is always maintained consistently.

This is preparation for more parameter destructuring adjustments. In particular, a follow-up CL will separate parameter recording from declaring the variables.

R=adamk@chromium.org, littledan@chromium.org
BUG=v8:811
LOG=N

Review URL: https://codereview.chromium.org/1259013003

Cr-Commit-Position: refs/heads/master@{#30002}