rust: Make NodePrefix allocation-free and Copy, remove NodePrefixRef...
rust: Make NodePrefix allocation-free and Copy, remove NodePrefixRef
The `*Ref` struct only existed to avoid allocating `Vec`s
when cloning `NodePrefix`, but we can avoid having `Vec`
in the first place by using an inline array instead.
This makes `NodePrefix` 21 bytes (with 1 for the length)
which is smaller than before as `Vec` alone is 24 bytes.
Differential Revision:
https://phab.mercurial-scm.org/D9863
