Historically, yacc has supported various kinds of inspections
and manipulations of the parser state, exposed as global variables.
The Go implementation of yacc puts that state (properly) in local
stack variables, so it can only be exposed explicitly.

There is now an explicit parser type, yyParser, returned by a
constructor, yyNewParser.

	type yyParser interface {
		Parse(yyLexer) int
		Lookahead() int
	}

Parse runs a parse. A call to the top-level func Parse
is equivalent to calling yyNewParser().Parse, but constructing
the parser explicitly makes it possible to access additional
parser methods, such as Lookahead.

Lookahead can be called during grammar actions to read
(but not consume) the value of the current lookahead token,
as returned by yylex.Lex. If there is no current lookahead token,
Lookahead returns -1. Invoking Lookahead corresponds to
reading the global variable yychar in a traditional Unix yacc grammar.

To support Lookahead, the internal parsing code now separates
the return value from Lex (yychar) from the reencoding used
by the parsing tables (yytoken). This has the effect that grammars
that read yychar directly in the action (possible since the actions
are in the same function that declares yychar) now correctly see values
from the Lex return value space, not the internal reencoding space.
This can fix bugs in ported grammars not even using SetParse and Lookahead.
(The reencoding was added on Plan 9 for large character sets.
No Plan 9 programs using yacc looked at yychar.)

Other methods may be added to yyParser later as needed.
Obvious candidates include equivalents for the traditional
yyclearin and yyerrok macros.

Change-Id: Iaf7649efcf97e09f44d1f5bc74bb563a11f225de
Reviewed-on: https://go-review.googlesource.com/4850Reviewed-by: Rob Pike <r@golang.org>