feat(template): transfer []byte type in load function

[xux1217]

when primary and foreign key is []byte, transfer []byte to string for map key.

This PR also fixes the binary foreign key error in the mysql test schema.

[stephenafamo]

For a tiny bit more efficiency, I'd prefer to detect the type during generation not at runtime. For example, take a look at how isPrimitive and usesPrimitive are used in the templates.