.[] doesn't cache a Seq

[zoffixznet]

Positional access on a Seq caches it. However, there's an edgecase where .[] doesn't cache it.

On List, .[] is a no-op and doesn't reify the List. I feel that the latter part can still apply to Seqs, but .[] should .cache, same as all other positional access would.

$ perl6 -e 'Nil for my $s := "abc".comb[*]; say $s'
(a b c)

$ perl6 -e 'Nil for my $s := "abc".comb[]; say $s'
The iterator of this Seq is already in use/consumed by another Seq
(you might solve this by adding .cache on usages of the Seq, or
by assigning the Seq into an array)
  in block <unit> at -e line 1

$ perl6 -v
This is Rakudo version 2018.06-146-g1b3dd35 built on MoarVM version 2018.06-141-gcae4731
implementing Perl 6.c.

[zoffixznet]

Tests went in in Raku/roast@7d23fbc

[zoffixznet]

Turns out fixing this Issue introduced a different inconsistency: [] caches, but <> doesn't. Since <> (and possibly [] too) are a common way to perform a decont operation and accidentally cached Seqs might be a hard memory-leak bugs to detect, I decided to revert the fix for this Issue and to spec that ZEN slicing does NOT cache Seqs.