Fix/parse national prefix with zero

[rubdos]

Follow-up from #92, since now @thvdveld has push access. Makes things a bit easier to collaborate on :-)

---

Fixes the parsing of phone numbers with a leading 0 after the country code, such as "+330631966543".

The parsing does not fail when doing:
parse(Some(country::FR), "+330631966543")

However, it does fail when the country code is not passed to it:
parse(None, "+330631966543")

When the country code is not passed to it, then the following block is not executed (which removes the national prefix when present):

    // Extract carrier and strip national prefix if present.
    if let Some(meta) = country.and_then(|c| database.by_id(c.as_ref())) {
        let mut potential = helper::national_number(meta, number.clone());


        // Strip national prefix if present.
        if let Some(prefix) = meta.national_prefix.as_ref() {
            if potential.national.starts_with(prefix) {
                potential.national = helper::trim(potential.national, prefix.len());
            }
        }


        if validator::length(meta, &potential, Type::Unknown) != Validation::TooShort {
            number = potential;
        }
    }

When parsing the number, we should find out what the country code is and then strip the national prefix, which is what this PR does. However, I'm not satisfied with how it turns out since detecting the country code is now duplicated. Normally, getting the country code is done with the Country helper struct that takes a &PhoneNumber.

This PR also adds a test case for the formatting of numbers with a national prefix.

Fixes #81

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.69%. Comparing base (1fd3d17) to head (e026c5e).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
- Coverage   67.81%   67.69%   -0.13%     
==========================================
  Files          18       18              
  Lines        2172     2139      -33     
==========================================
- Hits         1473     1448      -25     
+ Misses        699      691       -8     

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[thvdveld]

Italian leading zero is still broken, but to fix this we need to rewrite other parts of the implementation. I think it should be fine to merge this already fixing #81.