Remove dead function.
Substantially rewrite the handling of globals.

Previously, there was a special handler for GLOBALS which tried to keep that
array and actual variables in sync. This was hampered by the fact that several
other classes peeked into the contents of frames.

This patch does two things 1) it makes globals more efficient by moving to a
cell-indirection scheme that covers more cases than the previous version 2)
reduces the amount of jiggery-pokery between classes.

Benchmarks which use global variables normally can get speed-ups of about 50%;
those accessing the $GLOBALS array can be upto 3x faster. Even Richards gets a
7% boost.
Use API for accessing a dictionary rather than peeking directly.

This makes things easier to understand and allows us to make accesses to this
immutable dictionary elidable.
Absorb the KeyError, so that this function can be used as an elidable.

Update its use cases accordingly.
Merge pull request #79 from hippyvm/opt_globals

Opt globals