Fix a whole ton of methods on cached Seqs
Fix a whole ton of methods on cached Seqs

When Seq is cached, its $!list gets .DEFINITE and we can get
cached stuff from it. However, several methods do not check
for presence of $!list cache and assume the absence of $!iter means
the Seq has been consumed and there is no cache.

Fixes incorrect "Seq has been consumed; try caching" error for:
infix:<eqv>, .iterator, .Slip, .join, .List, .list, .eager,
.Array, .is-lazy, and .sink

Bug find: https://irclog.perlgeek.de/perl6-dev/2017-05-03#i_14524925

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