Support heredoc opened inside the evaluation

[hurricup]

E.g.

use v5.10;

my $text = 'test';
$text =~ s/test/<<EOM/e;
outer test
EOM

say $text

This require mandatory sublexing of lazy elements, so not sure if this is good performance-wise

[hurricup]

This sample works without parsing errors, but actually it is parsed incorrectly.

use v5.10;

say <<EOM;
outer ${\<<EM} text
INNER TEXT
EM
EOM

say $text