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

.github/workflows/main.yml

@@ -92,7 +92,6 @@ jobs:
     name: Test
     runs-on: ${{ matrix.os }}
     strategy:
-      fail-fast: false
       matrix:
         build: [stable, beta, nightly, windows, macos]
         include:
@@ -165,7 +164,6 @@ jobs:
     name: Python Wheel
     runs-on: ${{ matrix.os }}
     strategy:
-      fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
     steps:
@@ -262,7 +260,6 @@ jobs:
     name: Build wasmtime
     runs-on: ${{ matrix.os }}
     strategy:
-      fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
     steps: