Make values be backed by []byte

[arv]

This makes all but types.Type be backed by a []byte.

The motivation is to reduce the allocations and the work needed to be done when we read parts of a value (especially prolly trees).

Towards #2270

[arv]

@rafael-atticlabs Feel free to review at this point.

Things left to do

• [x] Make Hash() not allocate
• [ ] Maybe share the underlying storage for the offsets of all values in a chunk
• [ ] types.Type

All of this can be done in new PRs.

[arv]

jenkins: test this

[arv]

jenkins: test this

[arv]

Landed. More work will be done in follow ups.