Swap Notation for annotate_snippets::snippet::Snippet

[tirix]

This is an attempt at using annotate_snippets::snippet::Snippet for the API, however I'm having issues with ownership ~and lifetime~ of the strings returned in the Snippet structures, so this is still WIP.

[digama0]

What's the reason for using strfmt? Couldn't Diagnostic have a Display instance instead, that does e.g.

    match *diag {
        BadCharacter(span, byte) => {
            write!(f, "Invalid character (byte value {byte}); Metamath source files are limited to \
                      US-ASCII with controls TAB, CR, LF, FF)", byte = byte)?
        }

[tirix]

Thank you Mario!

• continuation passing style is exactly what I need for passing the snippets,
• also using the format! macro instead of strfmt, as you suggested,
• as we're getting all the diagnostics in a single function call, all generic T returned by CPS are collected in a Vec<T>,
• I'm using Cow's to pass the labels: either static or formatted strings,
• I using an Arena to ensure these live long enough (that's also an idea taken from the MM0 code)
• ~There are still some issues with line counting and span size, so this is still WIP for now~ Ready to merge!

[tirix]

This addresses one of the items of API issues #35.