Specialized handled for common Pattern/Enumerator combinations.

The common cases are:
var <- values-to-be enumerated 
or
type var <- values-to-be enumerated

Both cases are now handled by specialized code that only does a subtype
check and assignment.

This gives modest speed improvements. Average speedup is now nearly 21x.
Merge branch 'master' of https://github.com/cwi-swat/rascal.git