Revert "Made globals a mutable hash."

This reverts commit 54ac00c81d74c1c936e3f2b5054a1ed3d83b1f59.

Looks like everyone in support of going for immutable gloabals.
Reverting my commit to make globals mutable.
Implemented simpledict type.

The simpledict type exposes (hashof symbol Address) as a python object.
Useful for accessing globals etc.
Implemented built-in function globals()
Implemented built-in function compile.
Implemented built-in function exec.
Added simple test-case to test exec
Merge remote branch 'origin/master' into modules

Conflicts:
	base/python-desugar.rkt
Fixed errors after merge.

The bases is now a list. Updated code and simpledict classes
accordingly.
wraped the compiled code in with python-lib so that the builtins are available to it.
Implemented __import__
disabled replace-global-scope to make modules work correctly.
Forgot to add builtins/module.rkt earlier.
modify IMPL
add $sys module
revise comments
merge Anand __import__ with mine