Fix in enum type substitution
FIX: Making names available at module init
Fixes in propagating mutabilities
bitvector-like checks working .. kinda
Restoring subt at vardecl
Adding support for optional fields.
Fixes with optional args, mutability
Some more fixes
Making optional fields a type fields

  - to allow for parametric instantiation at setProp

  - Fixes at subtyping
A few more fixes with unification of option field
Merge branch 'master' into tsc_bench
Merge pull request #78 from UCSD-PL/tsc_bench

Several fixes and support for optional fields in object types