This is a huge change, which includes some changes to replace code that
originally cloned to reuse allocations instead. The updated
`elastic-array` crate renames its consuming `Vec`-conversion method to
`into_vec`, which means that I can do a simple
`sed -i 's/to_vec/into_vec/'` and then fix the compilation errors.
This commit is probably a minor performance win and definitely a
significant readability win.
* header back-references for on demand
* initial back-reference implementation for on demand requests
* answer requests from cache
* answer requests from cache, add tests
* strongly typed responses for vectors of homogeneous requests
* fix fallout in RPC without optimizing
