check error support pgx (#44)

pgstmt: fix values data type (#42)

pgstmt: impl select exists (#41)

change how IsErrorCode check error with errors.As (#40)

update dependencies

get sql state from interface (#38)

pgctx: add RunInReadOnlyTx (#34)

refactor: interface{} to any

pgstmt: union (#27)

* pgstmt: add union

* pgstmt: add join union to select statement

* add offset

* add nested union

* update test

pgctx: add key context (#25)

* pgctx: add key context

* adjust api

* add test

* refactor