Auto merge of #36551 - eddyb:meta-games, r=nikomatsakis

Auto merge of #36551 - eddyb:meta-games, r=nikomatsakis

Refactor away RBML from rustc_metadata.

RBML and `ty{en,de}code` have had their long-overdue purge. Summary of changes:
* Metadata is now a tree encoded in post-order and with relative backward references pointing to children nodes. With auto-deriving and type safety, this makes maintenance and adding new information to metadata painless and bug-free by default. It's also more compact and cache-friendly (cache misses should be proportional to the depth of the node being accessed, not the number of siblings as in EBML/RBML).
* Metadata sizes have been reduced, for `libcore` it went down 16% (`8.38MB` -> `7.05MB`) and for `libstd` 14% (`3.53MB` -> `3.03MB`), while encoding more or less the same information
* Specialization is used in the bundled `libserialize` (crates.io `rustc_serialize` remains unaffected) to customize the encoding (and more importantly, decoding) of various types, most notably those interned in the `TyCtxt`. Some of this abuses a soundness hole pending a fix (cc @aturon), but when that fix arrives, we'll move to macros 1.1 `#[derive]` and custom `TyCtxt`-aware serialization traits.
* Enumerating children of modules from other crates is now orthogonal to describing those items via `Def` - this is a step towards bridging crate-local HIR and cross-crate metadata
* `CrateNum` has been moved to `rustc` and both it and `NodeId` are now newtypes instead of `u32` aliases, for specializing their decoding. This is `[syntax-breaking]` (cc @Manishearth ).

cc @rust-lang/compiler