Implement automatic overloaded dereference.
Closes #7141.
Introduce a common recursion limit for auto-dereference and monomorphization.
Apply @nikomatsakis' nits and comments patch.
De-@ Session usage.
De-@ ty::ctxt usage.
De-@ tyencode::ctxt and related parts of astencode.
De-@ liveness.
De-@ type checking contexts.
De-@ trans contexts.
De-@ reachable.