* CoreDb: remove PHK tries
why:
There is no general use anymore for an MPT with a pre-hashed key. It
was used to resemble the `SecureHexaryTrie` logic from the legacy DB.
The only pace where this is needed is the `Leger` which uses a
a distinct MPT version anyway (see `distinct_ledgers.nim`.)
* Rename `CoreDx*` -> `CoreDb*`
why:
The naming `CoreDx*` was used to differentiate the new CoreDb API from
the legacy API which had descriptors named `CoreDb*`.
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