Fix various bugs in \(...) syntax.

It didn't correctly differentiate quoted pairs (positional arg) from
identifier paris (named arg), nor did it support | for flattening into
the Capture. This patches fixes both of these issues, as well as making
it handle ;s.

src/Perl6/Actions.nqp

@@ -3538,18 +3538,13 @@ class Perl6::Actions is HLL::Actions does STDActions {
     }
 
     method capterm($/) {
-        # Construct a Parcel, and then call .Capture to coerce it to a capture.
-        my $past := $<termish> ?? $<termish>.ast !!
-                    $<capture> ?? $<capture>.ast !!
-                    QAST::Op.new( :op('call'), :name('&infix:<,>') );
-        unless $past.isa(QAST::Op) && $past.name eq '&infix:<,>' {
-            $past := QAST::Op.new( :op('call'), :name('&infix:<,>'), $past );
-        }
-        make QAST::Op.new( :op('callmethod'), :name('Capture'), $past);
-    }
-
-    method capture($/) {
-        make $<EXPR>.ast;
+        my $past := $<termish>
+            ?? QAST::Op.new( $<termish>.ast )
+            !! $<semiarglist>.ast;
+        $past.unshift(QAST::WVal.new( :value($*W.find_symbol(['Capture']) ) ));
+        $past.op('callmethod');
+        $past.name('from-args');
+        make $past;
     }
 
     method multisig($/) {

src/Perl6/Grammar.nqp

@@ -2804,16 +2804,12 @@ grammar Perl6::Grammar is HLL::Grammar does STD {
     token capterm {
         '\\'
         [
-        | '(' <.ws> <capture>? ')'
+        | '(' <semiarglist> ')'
         | <?before \S> <termish>
         | {} <.panic: "You can't backslash that">
         ]
     }
 
-    rule capture {
-        <EXPR>
-    }
-
     rule param_sep {
         '' $<sep>=[','|':'|';;'|';'] { @*seps.push($<sep>) }
     }

src/core/Capture.pm

@@ -7,6 +7,8 @@ my class Capture { # declared in BOOTSTRAP
         nqp::create(self).BUILD(:@list,:%hash);
     }
 
+    method from-args(|c) { c }
+
     submethod BUILD(:@list, :%hash) {
         nqp::bindattr(self, Capture, '$!list',
             nqp::getattr(nqp::decont(@list.Parcel), Parcel, '$!storage')