Implement FromStr for zonemd Scheme and Algorithm

[mozzieongit]

While writing the dnst signzone zonemd argument parsing, I noticed that zonemd::Scheme and zonemd::Algorithm implement neither FromStr nor a from_mnemonic function (as with the enums created with the iana int_enum macro).
I was unsure which one should be used, but settled on FromStr with both number and mnemonic, as it allows an easy use of str::parse.

[ximon18]

After discussing internally I have reworked this to implement the ZONEMD IANA parameters using the IANA macros as we do for other IANA types and I have labelled this as a breaking change.

By using the macros we get a FromStr impl and to/from_mnemonic() functions, and consistency with the rest of domain.

This does however mean that types are renamed, moved in the module tree, and the "reserved", "unassigned" and "private" enum variants are gone (as we don't do that for other IANA parameters) and the ZonefileFmt impl no longer produces comments for the scheme (as it's a bit of a pain to code cleanly to match the way it was before with "unassigned", "private" and "reserved" ranges and without them there's only a single scheme so the comment has little value, nor does LDNS produce a comment for this type) and algorithm (as we don't output comments for the algorithm on other types).

[mozzieongit]

Noting it down, to not forget it: This (and other types in future PRs) should potentially change to use int_enum_str_with_decimal! instead of int_enum_str_decimal; the difference being that ..._with_decimal offers from_str with a mnemonic AND a number (as a string), while _str_decimal only allows using a number.