Accept Uint8Array in place of Buffer

[Gozala]

Changes to dag-pb attempted to add support for Uint8Arrays in place of node Buffer to better support case where data is passed across web workers. @achingbrain had a thought that it might be better yet to add support for Uint8Arrays at this layer instead.

Opening issue to track / discuss this here.

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[Gozala]

While I'm inclined to remove dependency on Buffer all together here, I do fear that this might break some assumptions in users of this library (72 on npm).

More pragmatic approach would be to just expect Uint8Array instance anywhere instead of Buffer instances, but keep return values to remain `Buffer.

[Gozala]

Is the assumption here that val is either node Buffer or a string ?

protons/src/compile/encodings.js

Lines 30 to 43 in 3b5276f

	var encode = function (val, buffer, offset) {
	var oldOffset = offset
	var len = bufferLength(val)
	varint.encode(len, buffer, offset)
	offset += varint.encode.bytes
	if (Buffer.isBuffer(val)) val.copy(buffer, offset)
	else buffer.write(val, offset, len)
	offset += len
	encode.bytes = offset - oldOffset
	return buffer
	}

Without necessarily understanding all the details here, it appears to me that supporting strings here is a bad idea. Because detecting byteLength would require one iteration and actual encoding would perform another iteration. Accepting just Uint8Arrays would avoid this because any strings would have need to be encoded first and byteLength info will be available right away.

This probably out of scope for this issue, but thought I'd still post it here.