Function generate_function 

pub fn generate_function(
    decl: &FunctionDecl,
    ctx: &ModuleContext,
    cgctx: Option<&CodegenContext<'_>>,
    doc: &Option<String>,
    scope: ScopeId,
) -> TokenStream

Generate wasm_bindgen extern blocks for a free function.

Expands optional params into multiple overloads, and generates try_ variants.