WIP : Implement a sortedmulti_a descriptor

[Harshit933]

This is mostly the copy of sortedmulti for now with some changes in the Terminal being MultiA instead of Multi and some max_satisfaction_witness_element and max_satisfaction_size as implemented in core.

Some Question that I have and I think we can implement it on the way in this PR.

• How can we build_template and what exactly is a template?
• How to calculate the script_size of sortedmulti_a.

Some other questions that I have about implementing sortedmulti_a inside tr like tr(c6...., sortedmulti_a(2, 5f...., 6e....). For now we build a taptree and consider the trees inside the tr like : tr(c6..., {pk(), pk()}. This means when I trying to build a tree for tr(c6...., sortedmulti_a(2, 5f...., 6e....) I am hitting an error with

`Result::unwrap()` on an `Err` value: Unexpected("sortedmulti_a(3 args) while parsing Miniscript")

One solution that I for now think is to implement a TrInner as we have inside Sh(). Am I on the right path?

[Harshit933]

@sanket1729 @apoelstra sorry for the ping. But can you please tell me if this is the right way of approaching the porblem or is there a better aproach to this?
Thanks.