`#[simd]` should work on functions without an explicit SIMD token parameter

Right now (#347), `#[simd]` requires the first non-receiver argument to a SIMD token. However, it's not uncommon for SIMD functions to not have an explicit SIMD token parameter, because any SIMD type (e.g. `f32xN`, etc) already carries a SIMD token with them. For example, this is what the `sRGB` example does:

https://github.com/linebender/fearless_simd/blob/bd9f7c7be6b4a2c009558fffd5db67c54ff744f5/fearless_simd/examples/srgb.rs#L52-L67

Such functions should be supported to make the `#[simd]` macro easier to use.

## Suggested solution

Instead of requiring that the first parameter ***is*** a SIMD token, require that the first parameter ***carries*** a SIMD token.

This could be done by adding a new trait like so:

```rs
pub trait ExtractToken { // bikeshed names
type S: Simd;
fn simd(&self) -> Self::S;
}

// all SIMD token types return self
impl ExtractToken for Avx2 {
type S = Self;
fn simd(&self) -> Self::S {
*self
}
}

// all data type return their SIMD token
impl ExtractToken for f32x8 {
type S = S;
fn simd(&self) -> Self::S {
self.simd
}
}
```

and change the code gen of the `#[simd]` macro to use this trait to get a SIMD token from the first parameter.

By adding [`#[diagnostic::on_unimplemented]`](https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-diagnosticon_unimplemented-attribute) on the trait, we can also get nice compiler errors when users use `#[simd]` incorrectly. E.g.

```rs
#[simd]
fn foo(a: i32);
```
```
error[E0277]: `i32` does not carry a SIMD token
--> src\encode\bc4.rs:961:24
|
961 | ExtractToken::simd(&a)
| ------------------ ^^ the trait `fearless_simd::ExtractToken` is not implemented for `i32`
| |
| required by a bound introduced by this call
|
= note: If you are using the #[simd] macro, the first parameter must carry a SIMD token. See the docs of #[simd] for more information.
help: the following other types implement trait `fearless_simd::ExtractToken`
--> ...
```

---

Note: I think the `ExtractToken` trait should be part of the public API of the main crate. I want to implement it on the newtype wrappers around `fearless_simd` types I use.

Start with the sRGB example at examples/srgb.rs lines 52–67 and compare the current #[simd] behavior described in issue #347. Trace the #[simd] macro implementation and determine how its first non-receiver argument is handled. Done means functions whose first parameter carries a SIMD token are accepted, while invalid parameters produce the intended diagnostic and the extraction trait is publicly usable.

Written by the indexing model from the issue text.