`Tezos_context` should export submodules rather than sub libraries

The following discussion from !8637 (merged) should be addressed:

@raphael-proust started a discussion: (+2 comments)
future improvement suggestion: Tezos_context should export submodules rather than sub libraries that have the full prefix in there.

Essentially, tezos_context.ml would become
```
module Disk = Tezos_context_disk
include Disk
module Memory = Tezos_context_memory
…
```