Add %precedence support.

Unfortunately it is not possible to reuse the %prec directive.  This
is because to please POSIX, we do not require to end the rules with a
semicolon.  As a result,

foo: bar %prec baz

is ambiguous: either a rule which precedence is that of baz, or a rule,
and then a declaration of the precedence of the token baz.

	* doc/bison.texinfo: Document %precedence.
	(Precedence Only): New.
	* src/assoc.h, src/assoc.c (precedence_assoc): New.
	* src/conflicts.c (resolve_sr_conflict): Support it.
	* src/scan-gram.l, src/parse-gram.y (%precedence): New token.
	Parse it.
	* tests/calc.at: Use %precedence for NEG.
	* tests/conflicts.at (%precedence does not suffice)
	(%precedence suffices): New tests.

tests/calc.at

@@ -106,11 +106,11 @@ static void unget_char (]AT_LEX_PRE_FORMALS[ int c);
 %token <ival> NUM "number"
 %type  <ival> exp
 
-%nonassoc '=' /* comparison	       */
+%nonassoc '='   /* comparison	       */
 %left '-' '+'
 %left '*' '/'
-%left NEG     /* negation--unary minus */
-%right '^'    /* exponentiation        */
+%precedence NEG /* negation--unary minus */
+%right '^'      /* exponentiation        */
 
 /* Grammar follows */
 %%