rust-nodemap: mutable NodeTree data structure
Thanks to the previously indexing abstraction,
the only difference in the lookup algorithm is that we
don't need the special case for an empty NodeTree any more.
We've considered making the mutable root an `Option<Block>`,
but that leads to unpleasant checks and `unwrap()` unless we
abstract it as typestate patterns (`NodeTree<Immutable>` and
`NodeTree<Mutated>`) which seem exaggerated in that
case.
The initial copy of the root block is a very minor
performance penalty, given that it typically occurs just once
per transaction.
Differential Revision: https://phab.mercurial-scm.org/D7793
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