* Rename `newKvt()` -> `ctx.getKvt()`
why:
Clean up legacy shortcut. Also, the `KVT` returned is not instantiated
but refers to the shared `KVT` that resides in a context which is a
generalisation of an in-memory database fork. The function `ctx`
retrieves the default context.
* Rename `newTransaction()` -> `ctx.newTransaction()`
why:
Clean up legacy shortcut. The transaction is applied to a context as a
generalisation of an in-memory database fork. The function `ctx`
retrieves the default context.
* Rename `getColumn(CtGeneric)` -> `getGeneric()`
why:
No more a list of well known sub-tries needed, a single one is enough.
In fact, `getColumn()` did only support a single sub-tree by now.
* Reduce TODO list
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