Consider a code generation type safe version of i18n-embed-fl

[kellpossible]

The https://github.com/baptiste0928/rosetta project brings some interesting ideas, it generates code using the build.rs script that can be used to reference localizations using native Rust types which also integrates well with IDE autocomplete. In addition, the function signature for localizations with variables are also generated, making it very easy to use a given localization correctly without needing to reference the source file. Seems like it should be possible to do something similar with fluent.

[kellpossible]

Perhaps this could be made into an optional feature/technique until it proves itself on some larger projects/examples.

[kellpossible]

This may rely on some ideas in #31 to be completely valid.

i18n::SOME_KEY.fmt(loader, arg1, arg2);
i18n::SOME_KEY.ATTRIBUTE.fmt(loader);

This does however lose some of the readability of the named arguments in i18n-embed-fl implementation.

[kellpossible]

Alternatively, messages can be functions which is what https://github.com/zaytsev/fluent-static seems to do, which seems like a much better idea.