There were some tokens used in the grammar but not declared. Antlr
doesn't really seem to care and happily uses them, but they appear in
RustLexer.tokens in a potentially-unexpected order.
This appears to not have too much of a detrimental effect, but it
doesn't seem to be what is intended either.
antlr doesn't mind that `PLUS` isn't declared in `tokens` and happily
uses the `PLUS` that appears later in the file, but the generated
RustLexer.tokens had PLUS at the end rather than where it was intended:
NOT=10
TILDE=11
PLUT=12
MINUS=13
...
PLUS=56
- Removes f128 from the grammar, which is no longer support in rustc
- The fragment modifier is added so it won't parse float suffix as a separate token
