Descriptors using H as a hardened indicator

[Ava Chow]

Hi All,

Recently someone pointed out that BIP 380 specifies that `H` can be used
as a hardened indicator in derivation paths, in addition to `'` and `h`.
However, there was also a test which suggested that `H` was not a valid
hardened indicator. A PR resolving this discrepancy by moving that test
from the invalid to the valid section was merged, but there is still a
question of whether this was the right course of action.

Currently, Bitcoin Core does not implement this, and AFAICT, neither
does rust-miniscript. There is an open PR to Bitcoin Core to allow `H`
(https://github.com/bitcoin/bitcoin/pull/32788), but ongoing discussion
suggests that perhaps the BIP should be modified instead as it seems
probable that no implementation of descriptors allows `H`.

So the question is: are there any implementations of descriptors that
emit descriptors that use `H` as a hardened indicator, or permit `H` as
a hardened indicator?

I think the former question is more useful to know as it is more
important that the descriptor implementations can parse each other's
products. It is possible that there are implementations that allow `H`
but do not emit `H`, since there are libraries that provide BIP 32
utilities which allow `H`.

Thanks
Ava