Add MIR type checker

[nikomatsakis]

Status update:

• [x] Skeleton of MIR type checker exists (added by #13, #52)
• [ ] Bring function where clauses into scope when creating initial environment
• [ ] Equate function signature with appropriate locals in type-check-goal/inputs-and-outputs
• [ ] In type-of/Place, generate goals for checking that the place's type is valid as we traverse
• [ ] In type-of/Rvalue, generate goals for checking that the rvalue is well-typed as we traverse
• [ ] Write the ty-signature function that finds the signature of a fn being invoked
• [ ] Tests for constants
• [ ] Tests for statics