parser: change LABEL type from pval to opval

parser: change LABEL type from pval to opval

The items pushed onto the parser stack can be one of several types:
ival, opval, pval etc. The only remaining use of pval is when a "label:"
is encountered.

When an error occurs during parsing, ops on the parse stack get
automatically reaped these days as part of the OP slab mechanism;
but bare strings (pvals) still leak.

Convert this one remaining pval into an opval, making the toker return
an OP_CONST with an SV holding the label.

Since newSTATEOP() still expects a raw string for the label, the parser
just grabs the value returned by the toker and makes a copy of the
string from it, then immediately frees the OP_CONST and its associated
SV.

The leak was showing up in ext/XS-APItest/t/stmtasexpr.t, which expects
to parse a statement where labels are banned.