Retain specific runtime APIs

[tadeohepperle]

fixes #933

I integrated the retaining of specific runtime APIs into the existing retain_metadata_pallets functionality, because it is more efficient to join the two operations in one place. As for the typing: I tried usind Option<FnMut(&str) -> bool> as a sensiple argument type for both pallets_filter and runtime_apis_filter but spent way too much time on lifetime issues and could not get it going well. So now I settled for just passing |_| true closures, when all elements should be retained. Not so nice, I know, but should work.

[jsdw]

The way to avoid the repetition would be by boxing the closures like so:

let retain_pallets_fn: Box<dyn Fn(&str) -> bool> = match opts.pallets.as_ref() {
    Some(pallets) => Box::new(|name| pallets.iter().any(|p| &**p == name)),
    None => Box::new(|_| true)
};
let retain_runtime_apis_fn: Box<dyn Fn(&str) -> bool> = match opts.runtime_apis.as_ref() {
    Some(apis) => Box::new(|name| apis.iter().any(|p| &**p == name)),
    None => Box::new(|_| true)
};
retain_metadata(&mut metadata_v15, retain_pallets_fn, retain_runtime_apis_fn);
metadata = metadata_v15.into();

A little extra overhead but not a big deal :)

[tadeohepperle]

Ah, nice, yeah that is definitely a solution. I still want to keep a check that at least one of the arguments is set, because if not we can avoid calling the retain_metadata function alltogether.

[jsdw]

Yeah I wouldn't reaplce the initial check, just the highlighted lines :)

[jsdw]

Nice work!

[lexnv]

Amazing PR! 👍