Remove cid.buffer in favor of cid.bytes.buffer to avoid issues with node libraries

[Gozala]

#29 repurposed former buffer: Buffer property to buffer: ArrayBuffer property that as @mikeal points out can lead to issues in node land that would happily accept either and mutate it.

This means that it could lead to issues when cid.byteOffset > 0 || cid.byteLength !== cid.buffer.byteLength. To avoid these problems we can rename .buffer to .arrayBuffer instead.

[Gozala]

@mikeal I recall your primary argument was that lot of node utils would take either and would write into ArrayBuffer just as if it was Buffer. However now that I think about it, writing into a cid.buffer would create whole class of other issues, so I'm no longer sue this is important.

I think we have also considered alternative of keeping buffer: ArrayBuffer and throwing if cid represents a view into slice of the ArrayBuffer. This got me thinking about following:

• We could emit warning and return a slice of this.arrayBuffer.slice(this.byteOffset, this.byteLength) in such case.
  • That would alleviate issues with node libraries.
  • It is also worth pointing out that CID will only be view into slice of ArrayBuffer when constructor is used instead of factory.