Consider updating Buffer.concat() to accept Uint8Array instances

[mbloch]

In v8.0.0, Node's Buffer.concat() function started accepting Uint8Arrays. It would be nice to add this behavior to the shim.

[dcousens]

s/Browser/Buffer?

[mbloch]

Silly me, fixed now.

[feross]

This will be released in 5.1.0