perf($parse): don't bind filters to a context

This change gives us ~10% boost in Chrome, less or nothing in other browsers.

BREAKING CHANGE:  `this` in filters is now undefined and no longer the scope

It's a bad practice for filters to have hidden dependencies, so pulling stuff from scope directly
is not a good idea. Scope being the filter context was never documented as public api, so we don't
expect that any significant code depends on this behavior.

If an existing filter has a dependency on the scope instance, the scope reference can
be passed into the filter as a filter argument (this is highly discouraged for new code):

Before: `{{ user.name | customFilter }}`
After: `{{ user.name | customFilter:this }}`