Refactor the types.rs types and structures (#681)

* Refactor the `types.rs` types and structures

A few changes applied along the way:

* Documentation added to most methods and types.
* Limits are now stored with the maximum as optional rather than a
  sentinel u32 value for `None`.
* The `Name` type was removed in favor of just using a bare `String`.
* The `Extern` prefix in the varaints of `ExternType` has been removed
  since it was redundant.
* Accessors of `ExternType` variants no longer panic, and unwrapping
  versions were added with "unwrap" in the name.
* Fields and methods named `r#type` were renamed to `ty` to avoid
  requiring a raw identifier to use them.

* Remove `fail-fast: false`

This was left around since the development of GitHub Actions for
wasmtime, but they're no longer needed!

* Fix compilation of the test-programs code

* Fix compilation of wasmtime-py package

* Run rustfmt

crates/misc/py/src/instance.rs

@@ -22,8 +22,8 @@ impl Instance {
         let exports = PyDict::new(py);
         let module = self.instance.borrow().module().clone();
         for (i, e) in module.borrow().exports().iter().enumerate() {
-            match e.r#type() {
-                wasmtime::ExternType::ExternFunc(ft) => {
+            match e.ty() {
+                wasmtime::ExternType::Func(ft) => {
                     let mut args_types = Vec::new();
                     for ty in ft.params().iter() {
                         args_types.push(ty.clone());
@@ -39,7 +39,7 @@ impl Instance {
                     )?;
                     exports.set_item(e.name().to_string(), f)?;
                 }
-                wasmtime::ExternType::ExternMemory(_) => {
+                wasmtime::ExternType::Memory(_) => {
                     let f = Py::new(
                         py,
                         Memory {