Provide a way to derive(Bake) on generic types without adding T: Bake bounds

[Manishearth]

A bunch of types (ZeroVec, VarZeroVec, etc) have impl<T> Bake for Foo<T> with no additional bounds, as they bake to a byte stream.

It should be possible to opt out of such bounds in derives containing such types, like CodePointTrie

[robertbastian]

I added the bounds on ZV on purpose, as I'd like to improve the format in the future (the zerovec! macro is a step in this direction).

In general it should be possible to opt out though, for example PhantomData<T> can be baked without T: Bake.

[sffc]

Discuss with:

• @Manishearth
• @sffc
• @robertbastian

[Manishearth]

Yeah, I agree with @robertbastian . In that case we probably need to just add the feature for phantomdata and such.

(unsure if we actually do need to discuss any more)