modeling functions, function types, and early/late-bound regions

[nikomatsakis]

#97 adds in a fn-def type that unifies early/late bound parameters into a single list. It also includes a better approach to implied bounds. But there's lots of work to do to handle fn-def types properly:

• Well-formedness rules for fn-def types #99
• Type-checking calls in the MIR #100
• Type-checking reification casts #101

[nikomatsakis]

@voidc was interested into looking into some of these tasks.