Consider TreeHashType for ProgressiveContainer #44

We have an MVP implementation of progressive containers and associated machinery in:

• ProgressiveMerkleHasher and progressive container support #43

Currently we are using TreeHashType::Container for progressive containers, because they behave essentially the same as containers, i.e. they are never packed.

At this stage, adding a new type just for progressive containers seems to be unnecessary work for no (?) benefit. It would require rippling changes into ssz_types (which is currently unchanged by progressive containers!) and milhouse, just for the sake of it. There is no benefit to, or need, in either library.

TreeHashType usage in ssz_types:

https://github.com/sigp/ssz_types/blob/6b615bf96e86926f45e19560d5ea052d38544218/src/tree_hash.rs#L24-L37

And milhouse:

https://github.com/sigp/milhouse/blob/4d88c724b265109823b801c89b17cbb1ddad3837/src/utils.rs#L68-L71

Mostly opening this issue so we have this decision documented and can revisit it in future if necessary.