Allow returning Result from functions

This commit adds support for exporting a function defined in Rust that returns a
`Result`, translating the `Ok` variant to the actual return value and the `Err`
variant to an exception that's thrown in JS.

The support for return types and descriptors was rejiggered a bit to be a bit
more abstract and more well suited for this purpose. We no longer distinguish
between functions with a return value and those without a return value.
Additionally a new trait, `ReturnWasmAbi`, is used for converting return values.
This trait is an internal implementation detail, however, and shouldn't surface
itself to users much (if at all).

Closes #841

crates/macro-support/src/parser.rs

@@ -726,13 +726,14 @@ impl<'a> MacroParse<(Option<BindgenAttrs>, &'a mut TokenStream)> for syn::Item {
                 }
                 let comments = extract_doc_comments(&f.attrs);
                 f.to_tokens(tokens);
+                let opts = opts.unwrap_or_default();
                 program.exports.push(ast::Export {
                     class: None,
                     method_self: None,
                     constructor: None,
                     comments,
                     rust_name: f.ident.clone(),
-                    function: f.convert(opts.unwrap_or_default())?,
+                    function: f.convert(opts)?,
                 });
             }
             syn::Item::Struct(mut s) => {