Remove BPCHAR to Swift Integer conversion

Removes support for converting PostgresData containing BPCHAR bytes (CHARACTER(n), CHAR, etc) to Swift integers. Postgres CHAR(n) fields may be padded with extra zero-bytes which makes conversion tricky. Since Postgres intends CHARACTER fields to store string values, PostgresData should only support conversion to Swift strings. To store 8-bits natively, use Postgres "char" type (note the quotes).