Integrate rustfmt to generator (#9)

### Context

This PR follows the recommendation in Zulip of doing what `ash` does: ash-rs/ash#735

Zulip thread: [#rust > A plan for SIMD @ 💬](https://xi.zulipchat.com/#narrow/channel/255911-rust/topic/A.20plan.20for.20SIMD/near/524498629)

### Changes

The first commit contains the logic changes to the CLI. The second commit contains `rustfmt`.

 - Added the missing `wasm` module such that running the CLI with no arguments regenerates WASM.
 - Added some documentation to the CLI and piped through `rustfmt`.
 - Ran the generator (getting formatted output).
 - Formatted the entire codebase.

### Test plan

Manually ran CLI. No logic changes in the generator (just formatting).

Author: ajakubowicz-canva

fearless_simd/src/core_arch/fallback.rs

@@ -13,4 +13,4 @@ impl Fallback {
     pub fn new() -> Self {
         Self { _private: () }
     }
-}
+}

fearless_simd/src/core_arch/mod.rs

@@ -6,6 +6,6 @@
 #[cfg(target_arch = "aarch64")]
 pub mod aarch64;
 
+pub mod fallback;
 #[cfg(target_arch = "x86_64")]
 pub mod x86_64;
-pub mod fallback;

fearless_simd/src/generated.rs

@@ -5,19 +5,19 @@
 //!
 //! All files in this subdirectory are autogenerated by the `fearless_simd_gen` crate.
 
+mod fallback;
 #[cfg(all(feature = "std", target_arch = "aarch64"))]
 mod neon;
-#[cfg(target_arch = "wasm32")]
-mod wasm;
-mod fallback;
 mod ops;
 mod simd_trait;
 mod simd_types;
+#[cfg(target_arch = "wasm32")]
+mod wasm;
 
+pub use fallback::*;
 #[cfg(all(feature = "std", target_arch = "aarch64"))]
 pub use neon::*;
-#[cfg(target_arch = "wasm32")]
-pub use wasm::*;
-pub use fallback::*;
 pub use simd_trait::*;
 pub use simd_types::*;
+#[cfg(target_arch = "wasm32")]
+pub use wasm::*;