Make FnScopes use hir::Expr

This was a bit complicated. I've added a wrapper type for now that does the
LocalSyntaxPtr <-> ExprId translation; we might want to get rid of that or give
it a nicer interface.

crates/ra_hir/src/lib.rs

@@ -47,7 +47,7 @@ pub use self::{
     ids::{HirFileId, DefId, DefLoc, MacroCallId, MacroCallLoc},
     macros::{MacroDef, MacroInput, MacroExpansion},
     module::{Module, ModuleId, Problem, nameres::{ItemMap, PerNs, Namespace}, ModuleScope, Resolution},
-    function::{Function, FnScopes},
+    function::{Function, FnScopes, ScopesWithSyntaxMapping},
     adt::{Struct, Enum},
     ty::Ty,
     impl_block::{ImplBlock, ImplItem},