This PR implements struct types, which will be serialized to JSON
before being sent to the mysql client using the mysql wire proto.

Internally, structs are just `map[string]interface{}`, but they can
actually be anything that's convertible to that, because of the way
the Convert method of the struct type is implemented. That means,
the result of an UDF or a table that returns a struct may be an
actual Go struct and it will then be transformed into the internal
map. It does have a penalty, though, because structs require
encoding to JSON and then decoding into `map[string]interface{}`.
Structs have a schema, identical to a table schema, except their
`Source` will always be empty.

Resolution of columns has also been slightly change in order to
resolve getting fields from structs using the `.` operator, which
required some trade-offs in some rules, such as not erroring
anymore in `qualify_columns` when the table is not found. That
error was delegated to `resolve_columns` in order to make resolution
possible, as the syntax is a bit ambiguous.
The advantage of using dot is the fact that no changes have to be made
to the parser in order for it to work.

Signed-off-by: Miguel Molina <miguel@erizocosmi.co>