Fix iterable.first(:end) on uncached iterators

.elems determines number of elements on an iterator without caching.
Prefix + caches the iterator.  The use of .elems caused:

  say "㊤".uniname.words.first(:end)

to fail.  ZoffixW++ for spotting

docs/ChangeLog

@@ -12,6 +12,7 @@ New in 2016.02
    + Specifying ::a now gives proper compile time error (RT #127504)
    + Rakudo jars are now installed in the proper location
    + Enums with a type object now fail with an NYI error
+   + .first(:end) on uncached iterators no longer fails
  + Additions:
    + Buf.unshift/Buf.prepend
    + DateTime.new(y,mo,d,h,mi,s) fully positional candidate

src/core/Any-iterable-methods.pm

@@ -698,15 +698,15 @@ Did you mean to add a stub (\{...\}) or did you mean to .classify?"
     proto method first(|) is nodal { * }
     multi method first(:$end) {
         $end
-          ?? ((my $elems = self.elems) ?? self.AT-POS($elems - 1) !! Nil)
+          ?? ((my $elems = +self) ?? self.AT-POS($elems - 1) !! Nil)
           !! ((my $x := self.iterator.pull-one) =:= IterationEnd ?? Nil !! $x)
     }
     multi method first(Bool:D $t) {
         fail X::Match::Bool.new( type => '.first' );
     }
     multi method first(Regex:D $test, :$end, *%a) is raw {
         if $end {
-            my $elems = self.elems;
+            my $elems = +self;
             if $elems && !($elems == Inf) {
                 my int $index = $elems;
                 return self!first-result($index,$_,'first :end',%a)
@@ -727,7 +727,7 @@ Did you mean to add a stub (\{...\}) or did you mean to .classify?"
     }
     multi method first(Callable:D $test, :$end, *%a is copy) is raw {
         if $end {
-            my $elems = self.elems;
+            my $elems = +self;
             if $elems && !($elems == Inf) {
                 my int $index = $elems;
                 return self!first-result($index,$_,'first :end',%a)
@@ -748,7 +748,7 @@ Did you mean to add a stub (\{...\}) or did you mean to .classify?"
     }
     multi method first(Mu $test, :$end, *%a) is raw {
         if $end {
-            my $elems = self.elems;
+            my $elems = +self;
             if $elems && !($elems == Inf) {
                 my int $index = $elems;
                 return self!first-result($index,$_,'first :end',%a)