Deduplicate in syntax::parse::token with a macro.

I also renumbered things at the same time; ``in`` was shifted into its
alphabetical position and the reserved keywords were reordered (a couple
of them were out of order).
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.)
Separate strict/reserved keywords, derive bounds.

It's twenty lines longer, but makes for clearer separation of strict and
reserved keywords (probably a good thing) and removes another moving
part (the definitions of `(STRICT|RESERVED)_KEYWORD_(START|FINAL)`).
auto merge of #10891 : chris-morgan/rust/macroize-(or-should-that-be-macroify)-syntax--parse--token-so-that-we-don't-make-mistakes-and-to-reduce-the-maintenance-burden, r=huonw

I also renumbered things at the same time; ``in`` was shifted into its
alphabetical position and the reserved keywords were reordered (a couple
of them were out of order).

Unused special identifiers are also removed in the second part.