Borrowing

[nikomatsakis]

Basic borrowing is in place, but more is needed:

• if the borrowed value lives in mutable memory, we need to increase (temporarily) the ref count of boxes (or else prohibit this mutable memory from being changed and/or prohibit the borrowing).
• evec/estr borrowing does not type check nor trans. Not sure why the former doesn't work but the latter is just unimplemented.

[boggle]

Awesome!

[nikomatsakis]

This is basically done, though there are no doubt lingering bugs remaining.