Remove the following unused special identifiers:

- underscore           ("_")
- unary                ("unary")
- not_fn               ("!")
- idx_fn               ("[]")
- unary_minus_fn       ("unary-")
- item                 ("item")
- block                ("block")
- stmt                 ("stmt")
- pat                  ("pat")
- expr                 ("expr")
- ty                   ("ty")
- ident                ("ident")
- path                 ("path")
- descrim              ("descrim")
- clownshoe_stack_shim ("__rust_stack_shim")
- blk                  ("blk")
- c_abi                ("C")

(And, of course, renumber everything to match.)