Implement calling built-in Functions

[stasm]

CallExpressions currently don't implement the ResolveValue trait. In the first iteration let's add a simple NUMBER() built-in which doesn't even take any additional arguments.

@zbraniecki What are our options for locale-aware number- and date-formatting?

[zbraniecki]

There are none. I have the fluent-locale crate almost ready which will add language negotiation and we can mock a simple PluralRules on it. I wanted to use it as the first builtin.

As for number/date formatting, I don't believe there's anything yet in Rust, but I expect things to start popping up soon. For now, we could just add a custom NUMBER function that transliterates to eastern arabic numerals if the language is ar.

[zbraniecki]

@stasm - I think this is already fixed - you can add custom functions and they can be called. We still await a decent implementation of either NUMBER or DATETIME to provide built-ins.

Do you want to keep this issue open until we find and implement those two?

[zbraniecki]

closing as per lack of activity. reopen if needed.

[Xiretza]

I think this should be reopened, the built-in functions are still not implemented, which is either a bug in fluent-rs or in https://projectfluent.org/fluent/guide/functions.html.

[alerque]

@Xiretza This issue has a lot of unrelated discussion about the implementation needed to have functions in the first place. I suggest opening a new issue (or issues) for the specific function(s) that need to be implemented to be a full spec implementation. That will make it easier to track. Thanks.