introduce hir debugging infra

This is to make debugging rust-analyzer easier.

The idea is that `dbg!(krate.debug(db))` will print the actual, fuzzy
crate name, instead of precise ID. Debug printing infra is a separate
thing, to make sure that the actual hir doesn't have access to global
information.

Do not use `.debug` for `log::` logging: debugging executes queries,
and might introduce unneded dependencies to the crate graph

crates/ra_hir/src/lib.rs

@@ -20,6 +20,7 @@ macro_rules! impl_froms {
 }
 
 mod either;
+pub mod debug;
 
 pub mod db;
 #[macro_use]