Add MIR type checker

[Niko Matsakis]

Status update:

• 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