Refactor to SwiftIdentifier

[tomlokhorst]

Implements first part of #136 (comment)

Future work:

• Add localized resources warnings
• Refactor warnings so that it's not an sideeffect anymore

Update this pattern:

let groupedResourceFiles = resourceFiles.groupBySwiftIdentifiers { $0.fullname }
groupedResourceFiles.printWarningsForDuplicatesAndEmpties(source: "resource file", result: "file")

// groupedResourceFiles.uniques.map(stuff)

To the following:

let groupedResourceFiles = resourceFiles.groupBySwiftIdentifiers(
  identifier: { $0.fullname }
  source: "resource file",
  result: "file"
)

// groupedResourceFiles.map(stuff)

[tomlokhorst]

Related bug I found while testing #235:

If there are duplicate keys in localized string files, a warning is printed, but the key isn't skipped for other localizations. This should be fixed when these two PRs are merged

• Duplicates in one locale should also skip other locales

Example where keys aren't correctly skipped:

// Localized.strings (en)
one = One;
one = One;

// Localized.strings (nl)
one = Een;