Does not yet support streaming, only strings and TypedArrays for now.

Adds a general HttpHeader flatbuffer message for serializing requests
and responses.

Also Buffer.readFrom in fetch() to buffer response.

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.

Pro:
time ./out/debug/deno tests/error_001.ts  3.0s -> 0.4s

Con:
time ./tool/build.py snapshot              33s -> 1m52s
out/debug/gen/snapshot_deno.bin            39M -> 121M

- Running repl from js side.
- Add tests for repl behavior.
- Handle ctrl-C and ctrl-D.

Do not confuse this with Node's Buffer. This is a direct port of Go's
bytes.Buffer - it allows buffering of Reader and Writer objects.

Initial implementation by Srijan Reddy (@srijanreddy98, #672).

Broken in master.

This reverts commit dd230d52.