Add borrow checker

[nikomatsakis]

I think we will base this design on polonius. There are various pieces to the borrow checker:

• [x] #84
• [ ] Region check -- detect errors from HRTB, or attempts to return data with type &'a u32 as a &'b u32, etc (this is mostly implemented in trait check, but we have to integrate it)
• [ ] Loan check -- figure out which paths are borrowed at each point and ensure accesses are consistent with those borrows