feat: streaming input decoding

[jetjinser]

Supports streaming bytes for parsing.

let inputs = [b"abc", b"\xf0", b"\x9f\x90\xb0"] // UTF8(🐰) == <F09F 90B0>
let decoder = @encoding.decoder(UTF8)
inspect!(decoder.decode!(inputs[0], stream=true), content="abc")
inspect!(decoder.consume!(inputs[1]), content="") // same with above
inspect!(decoder.consume!(inputs[2]), content="🐰")
assert_true!(decoder.finish!().is_empty())

[jetjinser]

ping @Yoorkin