introduce completion presentation

This module should remove completion rendering boilerplate from the
"brains" of completion engine.

crates/ra_hir/src/lib.rs

@@ -55,7 +55,7 @@ pub use self::{
     ids::{HirFileId, MacroCallId, MacroCallLoc, HirInterner},
     macros::{MacroDef, MacroInput, MacroExpansion},
     nameres::{ItemMap, PerNs, Namespace},
-    ty::Ty,
+    ty::{Ty, Substs},
     impl_block::{ImplBlock, ImplItem},
     docs::{Docs, Documentation},
     adt::AdtDef,