impl From for FontStretch, FontStyle

[dhardy]

Closes #808.

I added a CI test for usvg with all features since this was not present.

Note also:

• fontdb::Weight could include From<u16>, but this is relatively unimportant
• svgtypes::FontFamily could implement From<fontdb::FontFamily>; the reverse would need to be fontdb::FontFamily<'a>: From<&'a svgtypes::FontFamily> (omitted since this is a different repo)
• fontdb::Query<'a> cannot implement From<usvg::Font> (or &'a usvg::Font) due to lifetime of the list of families.

[dhardy]

Also note: the two enum conversions included here are the only bits I particularly wanted anyway.