&CORE::select()

&CORE::select()

This commit allows CORE::select to be called through references and
via &ampersand syntax.

This is a tricky case, as the select keyword represents two distinct
operators.  ck_select replaces the OP_SELECT with an OP_SSELECT if
there is more that one argument.

So what we do here is create an if(@_>1)/else block with the usual
op-with-coreargs-child inside each branch.

The op tree looks like this:

$ ./perl -Ilib -mO=Concise,CORE::select -e 'BEGIN{\&CORE::select}
'
CORE::select:
8  <1> leavesub[1 ref] K/REFC,1 ->(end)
-     <1> null K/1 ->8
5        <|> cond_expr(other->6) K/1 ->9
4           <2> gt sK/2 ->5
2              <1> rv2av[t4] sK/1 ->3
1                 <#> gv[*_] s ->2
3              <$> const[IV 1] s ->4
7           <@> sselect[t2] K ->8
-              <0> ex-pushmark s ->6
6              <$> coreargs(IV 218) ->7
a           <@> select[t1] sK/1 ->8
-              <0> ex-pushmark s ->9
9              <$> coreargs(IV 219) s/DREF1 ->a
-e syntax OK

There was no need to modify pp_select to handle a null when there is
no argument, as it can already handle it.