hush: support "f() (cmd)" functions

Many other shells support this construct

function                                             old     new   delta
parse_stream                                        2950    3018     +68

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-misc/func5.tests b/shell/hush_test/hush-misc/func5.tests
index 9c5f9fa..5c33560 100755
--- a/shell/hush_test/hush-misc/func5.tests
+++ b/shell/hush_test/hush-misc/func5.tests
@@ -1,9 +1,8 @@
 f() { echo $1; }
 f 1
 
-# hush fails on this syntax, but i've never seen anyone use it ...
-#f() ( echo $1; )
+f() ( echo $1; )
 f 2
 
-#f() ( echo $1 )
+f() ( echo $1 )
 f 3