Previously, EnvironmentRecord was a JS::Object. This was done because
GlobalObject inherited from EnvironmentRecord. Now that this is no
longer the case, we can simplify things by making EnvironmentRecord
inherit from Cell directly.
This also removes the need for environment records to have a shape,
which was awkward. This will be removed in the following patch.
This patch adds FunctionEnvironmentRecord as a subclass of the existing
DeclarativeEnvironmentRecord. Things that are specific to function
environment records move into there, simplifying the base.
Most of the abstract operations related to function environment records
are rewritten to match the spec exactly. I also had to implement
GetThisEnvironment() and GetSuperConstructor() to keep tests working
after the changes, so that's nice as well. :^)
