ts: Anchor clients for non anchor programs

[armaniferrante]

The coder/serialization interface should be pluggable so that one can simply create and generate an anchor client for non anchor programs--given an IDL for the non anchor program. The spl token program would be a great first test.

[leofisG]

This is interesting.

I wonder how will one go about specifying optional accounts in the IDL.

In SPL program, we have seen cases where the program performs different logic based on the number of accounts client pass into. Checking the IDL definition (here), it seems like all accounts are assumed to be required?

[Necmttn]

This is somehow related: #1176 with the little direction I can try to do it.

[armaniferrante]

Going to do the first part of this. That is, transitioning Coder into a common interface.