Consider new folder / package layout

- schemer
	- BoolSchema
	- ...
	- ObjectField
	- Schema
	- SchemaOptions
	- Register
- schemer/decoder
	- decoder.NewSchemer
	- decoder.NewJSON
	- decoder.SchemaOf
	- decoder.SchemaOfType
- schemer/util
	- PreEncode
	- PreDecode
	- ReadUvarint
	- WriteUvarint
	- StructTag