Support parametric types.

There are two issues to handle: first scanning nested angle bracket pairs
to support types such as std::pair< std::string, std::list<std::string> > >.

Another issue is to address idiosyncracies of C++: do not glue two closing
angle brackets together (otherwise it's operator>>), and avoid sticking
blindly a TYPE to the opening <, as it can result in '<:' which is a
digraph for '['.

	* src/scan-gram.l (brace_level): Rename as...
	(nesting): this.
	(SC_TAG): New.
	Implement support for complex tags.
	(tag): Accept \n, but not <.
	* data/lalr1.cc (b4_symbol_value, b4_symbol_value_template)
	(b4_symbol_variant): Leave space around types as parameters.
	* examples/variant.yy: Use nested template types and leading ::.
	* src/parse-gram.y (TYPE, TYPE_TAG_ANY, TYPE_TAG_NONE, type.opt):
	Rename as...
	(TAG, TAG_ANY, TAG_NONE, tag.opt): these.
	* tests/c++.at: Test parametric types.

data/lalr1.cc

@@ -72,7 +72,7 @@ b4_variant_if([
   # ----------------------------
   m4_define([b4_symbol_value],
   [m4_ifval([$2],
-            [$1.as<$2>()],
+            [$1.as< $2 >()],
             [$1])])
 
   # b4_symbol_value_template(VAL, [TYPE])
@@ -80,7 +80,7 @@ b4_variant_if([
   # Same as b4_symbol_value, but used in a template method.
   m4_define([b4_symbol_value_template],
   [m4_ifval([$2],
-            [$1.template as<$2>()],
+            [$1.template as< $2 >()],
             [$1])])
 ]) # b4_variant_if
 
@@ -366,7 +366,7 @@ m4_map([b4_symbol_constructor_definition_], m4_defn([b4_symbol_numbers]))])])
 # YYTYPE.
 m4_define([b4_symbol_variant],
 [m4_pushdef([b4_dollar_dollar],
-            [$2.$3<$][3>(m4_shift3($@))])dnl
+            [$2.$3< $][3 >(m4_shift3($@))])dnl
   switch ($1)
     {
 m4_map([b4_type_action_], m4_defn([b4_type_names]))[]dnl