Reimplement List.push using nqp::push()

The old way was most definitely not thread safe: this way, we have one less
place to look out for thread safety.

src/core/Array.pm

@@ -89,11 +89,6 @@ class Array {
         nqp::findmethod(List, 'REIFY')(self, parcel, nextiter)
     }
 
-    method STORE_AT_POS(Int \pos, Mu $v is copy) is rw {
-        nqp::bindpos(nqp::getattr(self, List, '$!items'),
-            nqp::unbox_i(pos), $v)
-    }
-
     method STORE(|) {
         # get arguments, shift off invocant
         my $args := nqp::p6argvmarray();

src/core/List.pm

@@ -183,9 +183,10 @@ my class List does Positional {
 
     multi method push(List:D: *@values) {
         fail 'Cannot .push an infinite list' if @values.infinite;
-        my $pos = self.gimme(*);
+        my $elems = self.gimme(*);
         fail 'Cannot .push to an infinite list' if $!nextiter.defined;
-        self.STORE_AT_POS($pos++, @values.shift) while @values.gimme(1);
+        nqp::bindattr( self, List, '$!items', nqp::list() ) if $elems == 0;
+        nqp::push( $!items, @values.shift ) while @values.gimme(1);
         self;
     }
 
@@ -371,10 +372,6 @@ my class List does Positional {
         parcel
     }
 
-    method STORE_AT_POS(Int \pos, Mu \v) is rw {
-        nqp::bindpos($!items, nqp::unbox_i(pos), v)
-    }
-
     method FLATTENABLE_LIST() { self.gimme(*); $!items }
     method FLATTENABLE_HASH() { nqp::hash() }