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/examples/memory.rs

@@ -151,7 +151,7 @@ fn main() -> Result<(), Error> {
     // Create stand-alone memory.
     // TODO(wasm+): Once Wasm allows multiple memories, turn this into import.
     println!("Creating stand-alone memory...");
-    let memorytype = MemoryType::new(Limits::new(5, 5));
+    let memorytype = MemoryType::new(Limits::new(5, Some(5)));
     let mut memory2 = Memory::new(&store, memorytype);
     check!(memory2.size(), 5u32);
     check!(memory2.grow(1), false);