Implement beginnings of generics

- add HIR for generic params
 - resolve generic params in type paths
 - add substitions for ADTs
 - insert type variables for substitutions

crates/ra_hir/src/lib.rs

@@ -24,6 +24,7 @@ mod type_ref;
 mod ty;
 mod impl_block;
 mod expr;
+mod generics;
 
 mod code_model_api;
 mod code_model_impl;