Move input.syntax to the corresponding chapter

We had a set of grammar rules we had placed in the notation chapter
under the label of "common productions".  There's a certain sense to
that, particularly if we had more of them.  As it is, though, all of
the productions seem related enough to the "input format" chapter, and
in fact we had already labeled these productions with the identifier
`input.syntax`, so let's go ahead and move these productions to the
"input format" chapter.  This looks more orderly, especially as we now
have rule labels in the notation chapter.

Author: traviscross

src/input-format.md

@@ -1,6 +1,19 @@
 r[input]
 # Input format
 
+r[input.syntax]
+```grammar,lexer
+@root CHAR -> <a Unicode scalar value>
+
+NUL -> U+0000
+
+TAB -> U+0009
+
+LF -> U+000A
+
+CR -> U+000D
+```
+
 r[input.intro]
 This chapter describes how a source file is interpreted as a sequence of tokens.
 

src/notation.md

@@ -50,27 +50,8 @@ r[notation.grammar.visualizations]
 
 Below each grammar block is a button to toggle the display of a [syntax diagram]. A square element is a non-terminal rule, and a rounded rectangle is a terminal.
 
-[syntax diagram]: https://en.wikipedia.org/wiki/Syntax_diagram
-
-r[notation.grammar.common]
-### Common productions
-
-The following are common definitions used in the grammar.
-
-r[input.syntax]
-```grammar,lexer
-@root CHAR -> <a Unicode scalar value>
-
-NUL -> U+0000
-
-TAB -> U+0009
-
-LF -> U+000A
-
-CR -> U+000D
-```
-
 [binary operators]: expressions/operator-expr.md#arithmetic-and-logical-binary-operators
 [keywords]: keywords.md
+[syntax diagram]: https://en.wikipedia.org/wiki/Syntax_diagram
 [tokens]: tokens.md
 [unary operators]: expressions/operator-expr.md#borrow-operators