Save upto 2 allocations per Date/DateTime.yyyy-mm-dd

- and dd-mm-yyyy and mm-dd-yyyy friends
- 1 Int allocation if month > 9
- 1 Int allocation if day > 9

For some reason, *in the core setting*, having a native int attribute
as part of a nqp::push_s (which should just convert it to a native
string), gives strange errors.  The workaround was to use nqp::to_str_I
on a nqp::getattr_i, which caused the Int allocation.  Oddly enough
using an native int attribute as part of a nqp::concat just works, so
use that together with an nqp::x in which the repeat factor is determined
by the result of the condition < 10 (which yields either 1 or 0).

src/core.c/Dateish.pm6

@@ -138,26 +138,22 @@ my role Dateish {
           ?? 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($parts,
+          nqp::concat(nqp::x('0',nqp::islt_i($!month,10)),$!month)
         );
-        nqp::push_s($parts, $!day < 10
-          ?? nqp::concat('0',$!day)
-          !! nqp::tostr_I(nqp::getattr_i(self,$?CLASS,'$!day'))
+        nqp::push_s($parts,
+          nqp::concat(nqp::x('0',nqp::islt_i($!day,10)),$!day)
         );
         nqp::join($sep,$parts)
     }
 
     method dd-mm-yyyy(str $sep = "-" --> Str:D) {
         my $parts := nqp::list_s;
-        nqp::push_s($parts, $!day < 10
-          ?? nqp::concat('0',$!day)
-          !! nqp::tostr_I(nqp::getattr_i(self,$?CLASS,'$!day'))
+        nqp::push_s($parts,
+          nqp::concat(nqp::x('0',nqp::islt_i($!day,10)),$!day)
         );
-        nqp::push_s($parts, $!month < 10
-          ?? nqp::concat('0',$!month)
-          !! nqp::tostr_I(nqp::getattr_i(self,$?CLASS,'$!month'))
+        nqp::push_s($parts,
+          nqp::concat(nqp::x('0',nqp::islt_i($!month,10)),$!month)
         );
         nqp::push_s($parts, $!year < 1000 || $!year > 9999
           ?? self!year-Str
@@ -168,13 +164,11 @@ my role Dateish {
 
     method mm-dd-yyyy(str $sep = "-" --> Str:D) {
         my $parts := nqp::list_s;
-        nqp::push_s($parts, $!month < 10
-          ?? nqp::concat('0',$!month)
-          !! nqp::tostr_I(nqp::getattr_i(self,$?CLASS,'$!month'))
+        nqp::push_s($parts,
+          nqp::concat(nqp::x('0',nqp::islt_i($!month,10)),$!month)
         );
-        nqp::push_s($parts, $!day < 10
-          ?? nqp::concat('0',$!day)
-          !! nqp::tostr_I(nqp::getattr_i(self,$?CLASS,'$!day'))
+        nqp::push_s($parts,
+          nqp::concat(nqp::x('0',nqp::islt_i($!day,10)),$!day)
         );
         nqp::push_s($parts, $!year < 1000 || $!year > 9999
           ?? self!year-Str