fix some plpgsql errors

[MGorkov]

This patch fixes the #243 , #160 , #248 issues and the following errors:
"variable "$1" does not exist"
"collations are not supported by type -"
"variable "ref" must be of type cursor or refcursor"
missing expression at or near ";"

changes in JSON:

1. add declared variable with "alliasfor" parameter pointing to associated variable, e.g. DECLARE _a ALIAS FOR $1;

    {
      PLpgSQL_var: {
        refname: '_a',
        aliasfor: '$1',
        lineno: 3,
        datatype: { PLpgSQL_type: { typname: 'text' } }
      }

2. add "retvarno" parameter for RETURN NEXT statement

    {
      PLpgSQL_stmt_return_next: { lineno: 6, retvarno: 2 }
    }

[lfittl]

Thanks for the patch, and conceptually looks good!

However it appears there is a problem with the protobuf-cpp tests here, blocking a merge.

I believe this is actually a larger issue with how we handle oneof in our JSON output, I started fixing that in #246, pulling in your commit there as well. Assuming I can work out the open issues on that PR I'll close this here and fix it by merging the other PR.