Avoid need for tink_<primitive>::init() methods

[daviddrysdale]

Investigate whether there's a safe way in Rust to have a global registry of KeyManager instances that are typed to a particular primitive (rather than the catchall enum Primitive).

See README for more details.

[daviddrysdale]

#60 adds a typed version of PrimitiveSet which allows the individual primitives to be typed (but doesn't alter the global registry nor the need for init() methods)