Allow constructing Sh descriptors wrapping an existing Wsh/Wpkh

[shesek]

Useful if you already have an W{s,pk}h descriptor and wish to wrap it with P2SH.

This used to be possible before the new inner types for the Descriptor enum variants, which has private fields that cannot be initialized externally.

[apoelstra]

ACK bd7c642

Looks good! I'll let @sanket1729 also check this out, in case he has opinions about the method naming.

[sanket1729]

ACK bd7c642

[sanket1729]

Actually, re-reading it. I have some complaints about the names :(.
This is not consistent with the other names in the API. We have names like Descriptor::new_wsh() instead of Descriptor::new_with_wsh. I would prefer to change to new_<name> from new_with_<name> without the with for consistency with other APIs.

We want to be more rust-idiomatic with names like Descriptor::with_wsh (instead of new_with_wsh (as suggested in this PR), or new_wsh as implemented in master). But that is another issue.

[shesek]

@sanket1729 Descriptor::new_sh_wsh(), Descriptor::new_sh_wpkh(), Sh::new_wsh() and Sh::new_wpkh() already exists though, with a different meaning (they accept the underlying Miniscript and wrap it with a W{s,pk}h descriptor, rather than accept the descriptor directly).