Create Whitespace grammar productions

This does not create any new productions, instead preferring comments.
rust-lang#1974 will involve pulling out the horizontal
whitespace into a separate production.

Comment wording (and casing) is modeled off of
https://www.unicode.org/reports/tr31/#R3a.
I left off a "unicode" prefix for ASCII items as they are likely common
enough in that context that specifying them as "unicode" could cause
more confusion.

Author: epage

src/input-format.md

@@ -6,12 +6,6 @@ r[input.syntax]
 @root CHAR -> <a Unicode scalar value>
 
 NUL -> U+0000
-
-TAB -> U+0009
-
-LF -> U+000A
-
-CR -> U+000D
 ```
 
 r[input.intro]

src/whitespace.md

@@ -1,21 +1,34 @@
 r[lex.whitespace]
 # Whitespace
 
+r[whitespace.syntax]
+```grammar,lexer
+@root WHITESPACE ->
+    // end of line
+      LF
+    | U+000B // vertical tabulation
+    | U+000C // form feed
+    | CR
+    | U+0085 // Unicode next line
+    | U+2028 // Unicode LINE SEPARATOR
+    | U+2029 // Unicode PARAGRAPH SEPARATOR
+    // Ignorable Code Point
+    | U+200E // Unicode LEFT-TO-RIGHT MARK
+    | U+200F // Unicode RIGHT-TO-LEFT MARK
+    // horizontal whitespace
+    | TAB
+    | U+0020  // space ' '
+
+TAB -> U+0009  // horizontal tab ('\t')
+
+LF -> U+000A  // line feed ('\n')
+
+CR -> U+000D  // carriage return ('\r')
+```
+
 r[lex.whitespace.intro]
 Whitespace is any non-empty string containing only characters that have the
-[`Pattern_White_Space`] Unicode property, namely:
-
-- `U+0009` (horizontal tab, `'\t'`)
-- `U+000A` (line feed, `'\n'`)
-- `U+000B` (vertical tab)
-- `U+000C` (form feed)
-- `U+000D` (carriage return, `'\r'`)
-- `U+0020` (space, `' '`)
-- `U+0085` (next line)
-- `U+200E` (left-to-right mark)
-- `U+200F` (right-to-left mark)
-- `U+2028` (line separator)
-- `U+2029` (paragraph separator)
+[`Pattern_White_Space`] Unicode property.
 
 r[lex.whitespace.token-sep]
 Rust is a "free-form" language, meaning that all forms of whitespace serve only