Redo PR rust-lang#24811 properly

Transplant the grammar-related changes to grammar.md,
and remove all grammar talk from reference.md

Author: Robin Kruppe

reference.md

@@ -65,14 +65,9 @@ explicit code point lists. [^inputformat]
   provided to the grammar verifier, restricted to ASCII range, when verifying the
   grammar in this document.
 
-## Special Unicode Productions
+## Identifiers
 
-The following productions in the Rust grammar are defined in terms of Unicode
-properties: `ident`, `non_null`, `non_eol`, `non_single_quote` and `non_double_quote`.
-
-### Identifiers
-
-The `ident` production is any nonempty Unicode[^non_ascii_idents] string of the following form:
+An identifier is any nonempty Unicode[^non_ascii_idents] string of the following form:
 
 [^non_ascii_idents]: Non-ASCII characters in identifiers are currently feature
   gated. This is expected to improve soon.
@@ -86,15 +81,6 @@ that does _not_ occur in the set of [keywords](#keywords).
 > character ranges used to form the more familiar C and Java language-family
 > identifiers.
 
-### Delimiter-restricted productions
-
-Some productions are defined by exclusion of particular Unicode characters:
-
-- `non_null` is any single Unicode character aside from `U+0000` (null)
-- `non_eol` is `non_null` restricted to exclude `U+000A` (`'\n'`)
-- `non_single_quote` is `non_null` restricted to exclude `U+0027`  (`'`)
-- `non_double_quote` is `non_null` restricted to exclude `U+0022` (`"`)
-
 ## Comments
 
 Comments in Rust code follow the general C++ style of line and block-comment