Adjust List!generate to always (lazily) flatten RPAs and Parcels

in the values of the list, even if no 'flatten' flag is set on
them.  This fixes empty [] to create an empty Array, as well as
reduce the number of places that we need to set the 'flatten' flag.

src/builtins/Array.pir

@@ -56,9 +56,6 @@ Arrays are the mutable form of Lists.
 .namespace []
 .sub '&circumfix:<[ ]>' 
     .param pmc values            :slurpy
-    .local pmc true
-    true = get_hll_global ['Bool'], 'True'
-    setprop values, 'flatten', true
     $P0 = new ['Array']
     $P0.'!STORE'(values)
     .return ($P0)

src/builtins/List.pir

@@ -119,50 +119,57 @@ Creates a List from its arguments.
 Ask the list to make at least the first C<n> elements as non-lazy
 as it can, then return its entire list of elements as a 
 ResizablePMCArray or Parcel.  If C<n> isn't given, then eagerly
-generate the entire list.
+generate the entire list.  Any RPAs (incl. Parcels) in $!values
+always flatten here, even if not marked with the 'flatten' flag.
 
 =cut
 
 .namespace ['List']
 .sub '!generate' :method
     .param int n               :optional
     .param int has_n           :opt_flag
-    .local pmc values, gen
+    .local pmc values
     values = getattribute self, '$!values'
     unless null values goto have_values
     values = new ['ResizablePMCArray']
     setattribute self, '$!values', values
   have_values:
-    gen    = getattribute self, '$!gen'
+    .local pmc gen
+    gen = getattribute self, '$!gen'
     unless null gen goto have_gen
     gen = box 0
     setattribute self, '$!gen', gen
   have_gen:
-  gen_loop:
-    .local int gen_i, values_i
+    .local int gen_i
     gen_i = gen
-    values_i = elements values
-    if has_n goto gen_elem
-    n = values_i
-  gen_elem:
-    unless gen_i < n goto gen_done
-    if gen_i >= values_i goto gen_done
+  gen_loop:
+    $I0 = elements values
+    if gen_i >= $I0 goto gen_done
     .local pmc elem
     elem = values[gen_i]
     if null elem goto gen_next
+    # RPAs (incl Parcel) always flatten
+    $I1 = isa elem, ['ResizablePMCArray']
+    if $I1 goto have_flat_elem
+    # see if we have a flattening element
     $P0 = getprop 'flatten', elem
     if null $P0 goto gen_next
+    # see if we need to flatten a certain number of elems
+    if has_n goto gen_elem_n
+    elem = elem.'!generate'()
+    goto have_flat_elem
+  gen_elem_n:
     $I0 = n - gen_i
-    $I1 = isa elem, ['ResizablePMCArray']
-    if $I1 goto have_flat_elem
     elem = elem.'!generate'($I0)
   have_flat_elem:
     splice values, elem, gen_i, 1
     goto gen_loop
   gen_next:
-    inc gen
-    goto gen_loop
+    inc gen_i
+    unless has_n goto gen_loop
+    if gen_i < n goto gen_loop
   gen_done:
+    gen = gen_i
     .return (values)
 .end
 

src/builtins/Parcel.pir

@@ -55,6 +55,9 @@ to a List.
     setattribute parcel, '$!values', values
     gen = box 0
     setattribute parcel, '$!gen', gen
+    # mark the List as a flattening object
+    $P0 = get_hll_global ['Bool'], 'True'
+    setprop parcel, 'flatten', $P0
     .return (parcel)
 .end
 
@@ -78,9 +81,6 @@ The canonical operator for creating a Parcel.
     parcel = new ['Parcel']
     transform_to_p6opaque parcel
     splice parcel, args, 0, 0
-    # mark the Parcel as flattening objects
-    $P0 = get_hll_global ['Bool'], 'True'
-    setprop parcel, 'flatten', $P0
     .return (parcel)
 .end