Implement the local JS snippets RFC

This commit is an implementation of [RFC 6] which enables crates to
inline local JS snippets into the final output artifact of
`wasm-bindgen`. This is accompanied with a few minor breaking changes
which are intended to be relatively minor in practice:

* The `module` attribute disallows paths starting with `./` and `../`.
  It requires paths starting with `/` to actually exist on the filesystem.
* The `--browser` flag no longer emits bundler-compatible code, but
  rather emits an ES module that can be natively loaded into a browser.

Otherwise be sure to check out [the RFC][RFC 6] for more details, and
otherwise this should implement at least the MVP version of the RFC!
Notably at this time JS snippets with `--nodejs` or `--no-modules` are
not supported and will unconditionally generate an error.

[RFC 6]: https://github.com/rustwasm/rfcs/pull/6

Closes #1311

crates/backend/src/ast.rs

@@ -2,6 +2,7 @@ use proc_macro2::{Ident, Span};
 use shared;
 use syn;
 use Diagnostic;
+use std::hash::{Hash, Hasher};
 
 /// An abstract syntax tree representing a rust program. Contains
 /// extra information for joining up this rust code with javascript.
@@ -24,6 +25,8 @@ pub struct Program {
     pub dictionaries: Vec<Dictionary>,
     /// custom typescript sections to be included in the definition file
     pub typescript_custom_sections: Vec<String>,
+    /// Inline JS snippets
+    pub inline_js: Vec<String>,
 }
 
 /// A rust to js interface. Allows interaction with rust objects/functions
@@ -66,11 +69,37 @@ pub enum MethodSelf {
 #[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq, Eq))]
 #[derive(Clone)]
 pub struct Import {
-    pub module: Option<String>,
+    pub module: ImportModule,
     pub js_namespace: Option<Ident>,
     pub kind: ImportKind,
 }
 
+#[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq, Eq))]
+#[derive(Clone)]
+pub enum ImportModule {
+    None,
+    Named(String, Span),
+    Inline(usize, Span),
+}
+
+impl Hash for ImportModule {
+    fn hash<H: Hasher>(&self, h: &mut H) {
+        match self {
+            ImportModule::None => {
+                0u8.hash(h);
+            }
+            ImportModule::Named(name, _) => {
+                1u8.hash(h);
+                name.hash(h);
+            }
+            ImportModule::Inline(idx, _) => {
+                2u8.hash(h);
+                idx.hash(h);
+            }
+        }
+    }
+}
+
 #[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq, Eq))]
 #[derive(Clone)]
 pub enum ImportKind {