switch json Hashes field to a map of bytes

[groob]

Right now we have the following structure:

// FileIntegrityMeta hashes and length of a file based resource to help ensure
// the binary footprint of the file hasn't been tampered with
type FileIntegrityMeta struct {
	Hashes map[hashingMethod]string `json:"hashes"`
	Length int                      `json:"length"`
}

The hashes are represented as a dictionary of strings, but it would be more accurate ( and easier to work with ) if we used a byte slice for each checksum.

Relevant from the notary types https://github.com/docker/notary/blob/c04e3e6d05881045def11167c51d4a8baa34899a/tuf/data/types.go#L169-L184