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.
De-@ filesearch.
De-@ CStore uses.
De-@ ParseSess uses.
De-@ resolve_lifetime.
De-@ typeck::check::method.
De-@ gather_loans.
De-@ move maps and rework parts of trans.
auto merge of #12735 : eddyb/rust/at-exodus-chapter-11, r=cmr