Layout: Accept Uint8Array type wherever the API expects a Buffer

Loosen Buffer type checks to accept the Uint8Array base class.
Uint8Array is preferred over Buffer due to its presence in both
Node.js and browser environments. When Buffer methods are needed,
create a new Buffer instance which uses the same underyling
memory of the Uint8Array parameter.

Signed-off-by: Justin Starry <justin@solana.com>