Fix some grammar inconsistencies for the '..' range notation.

Grammar changes:
* allow 'for _ in 1..i {}' (fixes rust-lang#20241)
* allow 'for _ in 1.. {}' as infinite loop
* prevent use of range notation in contexts where only operators of high
  precedence are expected (fixes rust-lang#20811)

Parser code cleanup:
* remove RESTRICTION_NO_DOTS
* make AS_PREC const and follow naming convention
* make min_prec inclusive

Author: dgrunwald

reference.md

@@ -3147,18 +3147,17 @@ The precedence of Rust binary operators is ordered as follows, going from
 strong to weak:
 
 ```{.text .precedence}
-* / %
 as
+* / %
 + -
 << >>
 &
 ^
 |
-< > <= >=
-== !=
+== != < > <= >=
 &&
 ||
-=
+= ..
 ```
 
 Operators at the same precedence level are evaluated left-to-right. [Unary