crux: compact sorted slice serialization

[skhal]

Serialize a sorted slice of paths in compact form by sharing prefix from the previous item found in Google codesearch:
https://github.com/google/codesearch/blob/b34f2a0c5ce12be3c9dc28038640afece6bee523/index/path.go#L123-L129

For example, a slice ["/foo/bar/a", "/foo/bar/b"] would store:

<a-size>/foo/bar/a\0<a-prefix-len><b-size>b\0

That is two data structures:

{
  size: 10,
  path: "/foo/bar/a",
}
{
  prefixSize: 9,
  size: 1
  path: "b"
}

When read it does the following (conceptually equivalent code):

var paths string[2]
paths[0] = "/foo/bar/a"
paths[1] = paths[0][:9] + "b"  // "/foo/bar/" + "b"

It allows to share prefix path between sorted items in the list.