Limit use of non-primitive types throughout the API

[dcousens]

As a result of the discussion in #359, the aim will be to limit the use of non primitives in the API.

Buffer is considered a primitive in this case, and is therefore acceptable.

[dcousens]

Wherever the API changes, warnings will need to be added to 1.5.x.

[dcousens]

Best I can tell, the only non-primitive constructor that we have is in function ECPair (d, Q, options).

[dcousens]

I didn't realise before, but Script is a major component which could be simplified down to a Buffer for interop.

With modules like https://github.com/bitcoinjs/bip69 coming up, the ability to reduce this part of the API would be asimilar to the changes made to Address.

And therefore highly beneficial.

[dcousens]

For the record, as per #407 (comment), script was transitioned to a Buffer in 2.0.0.
As was a Address to address.

Non-POD data types that still exist are basically limited to ECDSA system... and that's it?
I don't think we'll be able to break down ECPair in any way useful.

[dcousens]

Related 0030854

[dcousens]

We really want to avoid the use of bigi in the API for the next version, IMHO (or any equivalent big int)

[dcousens]

Closing in favour of #508