Use #[cfg(doc)] declarations to include WASM-specific functionality on native doc builds.

[ajrcarey]

Currently, when running cargo doc on a native platform, documentation for WASM-specific functionality is not included. (This is the case on docs.rs as well.)

Rework the inline documentation of WASM-specific items using #[cfg(doc)] declarations, so that cargo doc generates documentation for WASM-specific functionality as well.

[ajrcarey]

The generation of multi-target documentation in a single run of cargo doc is a long-standing problem; for context, see rust-lang/rust#1998 and rust-lang/rust#43348.

Recent builds of rustdoc pass a special cfg target setting, doc, that can be detected during documentation generation. We can change our current #[cfg(target_arch = "wasm32")] declarations to #[cfg(any(doc, target_arch = "wasm32"))] to have those functions included in our documentation builds.

[ajrcarey]

Certain types that are imported only when compiling to WASM - items from the js_sys and wasm_bindgen crates, for instance - must be available when generating documentation as well. Ideally, applying the doc target to Cargo.toml would automatically include the items we need, but this does not work; probably those crates are entirely wrapped with WASM-specific declarations, so even if we include them successfully with Cargo.toml, they are not exposed except when building on WASM. After thinking about it for a while, I decided the simplest solution was to create empty dummy structs like so:

#[cfg(doc)]
struct JsValue;

This is sufficient for the call to cargo doc to succeed, which is all that we need.

[ajrcarey]

Updated #[cfg()] declarations for all WASM-specific functions. Bumped crate version number to 0.7.23. Updated README.md.