Fix reading of BitString nodes

Fix reading of BitString nodes

The node tokenizer went out of its way to store BitString node values
without the leading 'b'.  But everything else in the system stores the
leading 'b'.  This would break if a BitString node is
read-printed-read.

Also, the node tokenizer didn't know that BitString node tokens could
also start with 'x'.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/4159834.1657405226@sss.pgh.pa.us