Publicise GenericParams

See https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0/topic/Exposing.20GenericParam.2Ename

crates/ra_hir/src/generics.rs

@@ -22,9 +22,9 @@
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct GenericParam {
     // FIXME: give generic params proper IDs
-    pub(crate) idx: u32,
-    pub(crate) name: Name,
-    pub(crate) default: Option<Path>,
+    pub idx: u32,
+    pub name: Name,
+    pub default: Option<Path>,
 }
 
 /// Data about the generic parameters of a function, struct, impl, etc.