Make Date/DateTime.yyyy-mm-dd about 250x as fast

Also allow a separator to be specified, so that you can now say:

    Date.yyyy-mm-dd("/") to get "2020/03/14" for today.

The default separator being "-", to remain compatible with earlier
versions.

It dawned upon me that:
 a. .yyyy-mm-dd was basically the default formatter of Date
 b. adding an optional separator would have close to no performance effects

src/core.c/Date.pm6

@@ -1,21 +1,6 @@
 my class Date does Dateish {
 
-    method !formatter(--> Str:D) {
-        my $parts := nqp::list_s;
-        nqp::push_s($parts, $!year < 1000 || $!year > 9999
-          ?? self!year-Str
-          !! nqp::tostr_I($!year)
-        );
-        nqp::push_s($parts, $!month < 10
-          ?? nqp::concat('0',nqp::tostr_I($!month))
-          !! nqp::tostr_I($!month)
-        );
-        nqp::push_s($parts, $!day < 10
-          ?? nqp::concat('0',nqp::tostr_I($!day))
-          !! nqp::tostr_I($!day)
-        );
-        nqp::join('-',$parts)
-    }
+    method !formatter(--> Str:D) { self.yyyy-mm-dd }
 
 #?if moar
     my constant $valid-units = nqp::hash(

src/core.c/Dateish.pm6

@@ -132,8 +132,21 @@ my role Dateish {
           + ($!month > 2 && IS-LEAP-YEAR($!year));
     }
 
-    method yyyy-mm-dd(--> Str:D) {
-        sprintf '%04d-%02d-%02d',$!year,$!month,$!day
+    method yyyy-mm-dd(str $sep = "-" --> Str:D) {
+        my $parts := nqp::list_s;
+        nqp::push_s($parts, $!year < 1000 || $!year > 9999
+          ?? self!year-Str
+          !! nqp::tostr_I(nqp::getattr_i(self,$?CLASS,'$!year'))
+        );
+        nqp::push_s($parts, $!month < 10
+          ?? nqp::concat('0',$!month)
+          !! nqp::tostr_I(nqp::getattr_i(self,$?CLASS,'$!month'))
+        );
+        nqp::push_s($parts, $!day < 10
+          ?? nqp::concat('0',$!day)
+          !! nqp::tostr_I(nqp::getattr_i(self,$?CLASS,'$!day'))
+        );
+        nqp::join($sep,$parts)
     }
 
     method earlier(*%unit) { self.later(:earlier, |%unit) }