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/test-programs/tests/wasm_tests/runtime.rs

@@ -68,9 +68,9 @@ pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> any
         .imports()
         .iter()
         .map(|i| {
-            let module_name = i.module().as_str();
+            let module_name = i.module();
             if let Some(instance) = module_registry.get(module_name) {
-                let field_name = i.name().as_str();
+                let field_name = i.name();
                 if let Some(export) = instance.find_export_by_name(field_name) {
                     Ok(export.clone())
                 } else {