This to-be-stable attribute is equivalent to `#[lang = "oom"]`.
It is required when using the alloc crate without the std crate.
It is called by `handle_alloc_error`, which is in turned called
by "infallible" allocations APIs such as `Vec::push`.

Remove emulation of hygiene with gensyms

Namely: labels, type parameters, bindings in patterns, parameter names in functions without body.
All of these do not need hygiene after lowering to HIR, only span locations.

Michael Woerister pointed out that `hir_to_node_id` (introduced in
August 2017's 28ddd7a4) supersedes the functionality of
`find_node_for_hir_id` (just a hash lookup compared to that linear
search).