feat: Make `FieldMap` usage more ergonomic

[icorderi]

Breaking changes:

• FieldMap::{UPnP, UUID, URN, Unknown} now hold String instead of Vec<u8>
• FieldMap::new now takes Into<Cow<'a, str>> instead of a &[u8]

Refactored:

• added FieldMap::parse_bytes with the same behavior as the old FieldMap::new

Additions:

• added constructors on FieldMap for each variant:
  • FieldMap::upnp(..) -> Option<FieldMap::UPnP>
  • FieldMap::uuid(..) -> Option<FieldMap::UUID>
  • FieldMap::urn(..) -> Option<FieldMap::URN>
  • FieldMap::unknown(..) -> Option<FieldMap::Unknown>

[coveralls]

Coverage decreased (-32.2%) to 58.924% when pulling 779691e30f6c80fb1e76e6867fbe4f70c6584bf4 on icorderi:refactor/fieldmap into ea3ce7bac7895125f48ab23c15ed5bd44c5acf43 on GGist:master.

[coveralls]

Coverage increased (+0.8%) to 58.752% when pulling de09c1a3315bc32bd380a503e93335df1a944fbc on icorderi:refactor/fieldmap into c86b63f5c4c3e5b33ce9c4aafd8a3a7301357fb5 on GGist:master.

[coveralls]

Coverage increased (+0.9%) to 55.711% when pulling 7c0604ff565aeea7051b49ff49388bc008850e3c on icorderi:refactor/fieldmap into b8b2e29e018ad19cb8d2aed6ccd6c6e66fa0bb18 on GGist:master.

[coveralls]

Coverage increased (+0.9%) to 55.711% when pulling 31100961b4115e3e6e551081f69d46a9619b443e on icorderi:refactor/fieldmap into 479c7170d83c84e50c50dfed91a979da5f4ecc79 on GGist:master.

[icorderi]

@GGist, do you want to pump the version of the library and publishing it to crates.io before merging this?

[GGist]

@icorderi Sorry for the late reply. I went ahead and bumped the minor version for the current changes. Will merge this in a bit.

[icorderi]

@GGist, btw, you bumped the minor without resetting the revision. We went from 0.1.6 to 0.2.6 hehe

[GGist]

Oh thanks for that. I have done that so many times and every time I tell myself it won't happen again hah.

Anyway, luckily this merge warrants a minor version bump as well so I can reset the patch version back to zero soon.