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rust-node: handling binary Node prefix

Parallel to the inner signatures of the nodetree functions in
revlog.c, we'll have to handle prefixes of `Node` in binary
form.

Another motivation is that it allows to convert from full Node
references to `NodePrefixRef` without copy. This is expected to
be by far the most common case in practice.

There's a slight complication due to the fact that we'll be sometimes
interested in prefixes with an odd number of hexadecimal digits,
which translates in binary form by a last byte in which only the
highest weight 4 bits are considered. This is totally transparent for
callers and could be revised once we have proper means to measure
performance.

The C implementation does the same, passing the length in nybbles as
function arguments. Because Rust byte slices already have a length, we carry
the even/odd informaton as a boolean, to avoid introducing logical
redundancies and the related potential inconsistency bugs.

There are a few candidates for inlining here, but we refrain from
such premature optimizations, letting the compiler decide.

Differential Revision: https://phab.mercurial-scm.org/D7790

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