subarray is not re-defined in index.d.ts

[BTOdell]

The subarray function should return a Buffer, but currently the types are inheriting the definition from Uint8Array which has the function returning a Uint8Array.
I believe PR #261 fixed this issue at runtime, but the types were never updated.

[cmdcolin]

ill just note buffer@4.9.2 (used by create-react-app@4 with webpack@4) returns a UInt8Array for subarray