There's a lot of stuff wrong with the representation of these types:
TyFnDef doesn't actually uniquely identify a function, TyFnPtr is used to
represent method calls, TyFnDef in the sub-expression of a cast isn't
correctly reified, and probably some other stuff I haven't discovered yet.
Splitting them seems like the right first step, though.
resolve_lifetime.rs: Switch from BlockScope to FnScope in ScopeChain
construction. Lifetimes introduced by a fn signature are scoped to the
call-site for that fn. (Note `add_scope_and_walk_fn` must only add
FnScope for the walk of body, *not* of the fn signature.)
region.rs: Introduce new CodeExtentData::CallSiteScope variant. Use
CodeExtentData as the cx.parent, rather than just a NodeId. Change
DestructionScopeData to CallSiteScopeData.
regionck.rs: Thread call_site_scope via Rcx; constrain fn return
values.
(update; incorporated review feedback from niko.)
