Use PL_parser->lex_shared instead of Sv[IN]VX(PL_linestr)

Use PL_parser->lex_shared instead of Sv[IN]VX(PL_linestr)

Unfortunately, PL_parser->linestr and PL_parser->bufptr are both
part of the API, so we can’t just move them to PL_parser->lex_shared.
Instead, we have to copy them in sublex_push, to make them visible to
inner lexing scopes.

This allows the SvIVX(PL_linestr) and SvNVX(PL_linestr) hack to
be removed.

It should also speed things up slightly.  We are already allocating
PL_parser->lex_shared in sublex_push, so there should be no need to
upgrade PL_linestr to SvNVX as well.

I was pleasantly surprised to see how the here-doc code seemed to
shrink all by itself when modified to account.

PL_sublex_info.super_bufptr is also superseded by the addition of
->ls_bufptr to the LEXSHARED struct.  Its old values when localised
were not visible, being stashed away on the savestack, so it was
harder to use.