String descriptors can panic on `to_string`

If you have a descriptor with String keys, and you parse one with a key outside of the checksum alphabet, desc_checksum will fail, leading to returning an error from Display::fmt, which apparently causes a panic in .to_string().

I'm not sure why we haven't seen this before but when fuzzing locally it now shows up quickly on the roundtrip_descriptor fuzztest. For example you can give it the test vector

do_test("pkh(\u{16})".as_bytes());

I'm unsure what the best plan of action here ... String descriptors aren't standardized, so I guess we can just make something up. The two ideas that come to mind are:

• Enforce, when parsing descriptors, that every character lands in the checksum alphabet (or at least, is ASCII-printable, which I think might be the same thing)
• When computing checksums, skip invalid characters or replace them with x or something
• Keep the existing behavior (though then we'd need to tweak our fuzztests somehow)

Also affects elements-miniscript.

Start with desc_checksum and the roundtrip_descriptor fuzztest, using the provided pkh(\u{16}) test vector to reproduce the Display::fmt failure. Compare the proposed parsing and checksum behaviors, including the impact on elements-miniscript; done means an agreed behavior no longer causes to_string() to panic and the fuzztest reflects it.