Add projection functions to Yoke

[sffc]

There are several use cases where we need to project a Yoke from one Yokeable type to another Yokeable type but keeping the same Cart; i.e., map Yoke<A, C> to Yoke<B, C>.

A few examples of use cases:

1. Yoke<ZeroMap<'static, Foo, Bar>, C> to Yoke<Bar, C> (getting a value from a map)
2. Yoke<&'static [u8], C> to Yoke<T, C> (applying a deserializer to a slice)
3. Yoke<T, C> to Yoke<Q, C> where Q is a field of T (selecting more specific data)

I can envision these operations being chained and performed by different data providers. For example, the single-file data provider can return DataPayload<BytesMarker>, which a deserialization provider converts to a DataPayload<LargerDataStructMarker>, which a data filtering provider converts to a DataPayload<SmallerDataStructMarker>.

CC @Manishearth

[Manishearth]

I think this can be done safely in a generic fashion but I'm not 100% sure and I need to think about it to figure out the correct bounds.