[es6] Some renamings and minor clean-ups in parameter parsing

[es6] Some renamings and minor clean-ups in parameter parsing

In particular, rename FormalParameterParsingState and friends to FormalParameters etc.

This should not change any logic, but is a preparatory CL for a bunch of follow-up fixes and clean-ups.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1247443004

Cr-Commit-Position: refs/heads/master@{#29807}