Saner approach to lvalues and callable values in trans

LValues no longer carry information about generics and objs, instead
there's an extended form of lvalue, lval_maybe_callee, only used by
call and bind, which holds this info.

This makes it possible to take the value of a method and get a working
closure, and will (with some more work) allow us to call statically
known functions without loading from their pair.

Closes rust-lang#435
Closes rust-lang#758

Author: marijnh