Allow mapping HashMap or BTreeMap

[ctron]

Motivation

Having to deal with a structure of:

export type State = {|
  styles: {|
    [key: string]: $Shape<CSSStyleDeclaration>,
  |},
  attributes: {|
    [key: string]: { [key: string]: string | boolean },
  |}
}

It would be helpful to have support, mapping this to a map type, like HashMap or BTreeMap.

Proposed Solution

Implement IntoWasmAbi for HashMap, BTreeMap, or both.

[ChristianIvicevic]

My use case is that I initially tried to use the following pattern which isn't supported by wasm-bindgen at the moment:

enum ItemKind {
    VariantOne,
    VariantTwo,
}

pub fn consume_from_js(values: impl Into<HashMap<ItemKind, usize>>) {}

My current workaround is to use an automatically generated struct based on the enum that behaves like an exhaustive HashMap using a bunch of derive_builder magic and has the following shape:

struct ItemKindStruct {
    variant_one: Option<usize>,
    variant_two: Option<usize>,
}

Without this the most "idiomatic" approach is most likely to work with JsValue across the boundary and trying to (de)serialize it, however it's not type-safe at the JS call-site and having to handle parsing in the Rust code is also inconvenient. Therefore I'd be really happy to see at least HashMap being supported at some point if possible.