use index tag first, field name second

[alovak]

When we marshal/unmarshal field values, we match struct fields by name first and by index tag second. When we check the field name we use regexp: ^F\d+$ (field name starts with F and has digits after it). Because composite fields can have tag names that are not digits, we only check that field starts with F with the regexp: ^F.+$.

So, if you marshal/unmarshal composite field that starts with F and also has index, then the index tag will be ignored. Imagine you have tag 01 and a struct:

type OriginalElements struct {
        ForwadingInsitutionCode *field.String `index:"01"`
}

Guess what? You will spend time trying to understand why tag 01 is not set when you marhsal/unmarshal data. And it's because the composite field will take orwadingInsitutionCode as an index of the field and will try to find a spec for such field.

To avoid such cases, we are changing the field name matching priorities by checking the index tag first and the field name second.

Technically, it can be a breaking change if you use index tag on the struct for something else.

[adamdecaf]

While this is a breaking change this PR resolves a pretty annoying bug that would take a while for someone to investigate, so it makes sense to fix. Also the original change is ~2 weeks old in this repository so few people have likely adopted it.