Fixes some sed errors introduced in c43cfe.
Unfortunately moving libdeno required splitting build.rs into two parts,
one for cli and one for core.
I've also removed the arm64 build - it's complicating things at this
re-org and we're not even testing it. I need to swing back to it and get
tools/test.py running for it.
Because many Reader/Writer implementations (e.g. bufio) assume their
able to use subarray() with byte indexes and often ask for byte values,
it makes sense to simply restrict all implementations to Uint8Array.
