Thunk optimization.

[V0ldek]

The current version of Harper.Interpreter.Thunk engine creates many unnecessary thunks. To list some:

• Wrapping a thunk into a thunk is unnecessary.
• Wrapping any object expression other than variables into a thunk is unnecessary.
• Wrapping literals into thunks is unnecessary.
  etc.

Most of these can be optimized away. First we need to identify a list of cases when thunks are absolutely necessary and then make sure that we only wrap these cases into thunks.