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