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/api/src/lib.rs

@@ -1,4 +1,10 @@
-//! Wasmtime embed API. Based on wasm-c-api.
+//! Wasmtime's embedding API
+//!
+//! This crate contains a high-level API used to interact with WebAssembly
+//! modules. The API here is intended to mirror the proposed [WebAssembly C
+//! API](https://github.com/WebAssembly/wasm-c-api), with small extensions here
+//! and there to implement Rust idioms. This crate also defines the actual C API
+//! itself for consumption from other languages.
 
 #![allow(improper_ctypes)]