Change goyacc to be reentrant.

Instead of calling the package scope Lex function,
Parse now takes an argument which is used to
do the lexing.
I reverted to having the generated switch
code inside Parse rather than a separate function because
the function needs 7 arguments or a context structure,
which seems unnecessary.
I used yyrun(), not the original $A so that
it's possible to run the backquoted code through gofmt.

R=rsc, ken2, ken3
CC=golang-dev
https://golang.org/cl/879041

src/cmd/goyacc/doc.go

@@ -19,5 +19,18 @@ The file units.y in this directory is a yacc grammar for a version of
 the Unix tool units, also written in Go and largely transliterated
 from the Plan 9 C version.
 
+The generated parser is reentrant. Parse expects to be given an
+argument that conforms to the following interface:
+
+	type yyLexer interface {
+		Lex(lval *yySymType) int
+	}
+
+Lex should return the token identifier, and place other token
+information in lval (which replaces the usual yylval).
+
+Code inside the parser may refer to the variable yylex
+which holds the yyLexer passed to Parse.
+
 */
 package documentation