Check and validate :result-types more thoroughly

[dieggsy]

Right now, you can pass seemingly arbitrary types to :result-types, with only :string and t actually having any effect. We should probably actually check the passed types for validity.

For example, in clsql-odbc, it doesn't make sense to ask for an int as an :unsigned-byte-vector, but we will happily move forward and just return it as an int in this case.

[dieggsy]

This is actually an issue in more than just clsql-odbc. For example clsql-sqlite3 allows specifying an :int :result-type for a text field.