The approach taken in this CL is to incrementally move toward the
currently-specced version of modules in ES6. The biggest change in this
patch is separating the parsing of modules from the parsing of scripts,
getting rid of the 'module' keyword and thus disallowing modules-in-scripts
as well as modules-in-modules.

The syntax supported by import/export declarations has not yet been significantly
changed, with the major exception being that import declarations require a string
as the 'from' part.

Most of the existing tests have been disabled, with a first new test added
in cctest/test-parsing.

BUG=v8:1569
LOG=n

Review URL: https://codereview.chromium.org/881623002

Cr-Commit-Position: refs/heads/master@{#26299}