Add conversion prim
Add immutibility annotation for strings and vectors
Fix erroneous test
* Cannot impersonate immutable vectors (i.e. those made with vector literals)
Closer to a proper implementation of impersonator-of?/chaperone-of?
Merge branch 'master' of https://github.com/samth/pycket

Conflicts:
	pycket/prims/equal.py
	pycket/prims/impersonator.py