Improved Conversion traits on Table Primary Keys

[MinisculeGirraffe]

This is a bit of a paper cut I noticed.

When defining an entity in the primary_key function, it performs a conversion from the KeyInput to the associated tables PrimaryKey.

In my code I really wanted to be able to do write a generic function that accepted any type that could be represented as that the primary key for the table associated with an entity.

fn example_signature<T: Entity>(input: impl Into<<<T as Entity>::Table as Table>::PrimaryKey>)

Instead of manually implementing the Into trait for my key inputs, it could just be implemented automatically for every type with the following trait bounds. But it would have to come from this library as the Entity and Table traits are defined here.

type KeyInput<'a, T: Entity> = <T as Entity>::KeyInput<'a>;
type PrimaryKey<T: Entity> = <<T as Entity>::Table as Table>::PrimaryKey;

impl<'a, T: Entity> From<KeyInput<'a, T>> for PrimaryKey<T> {
    fn from(value: <T as Entity>::KeyInput<'a>) -> Self {
        T::primary_key(value)
    }
}

The utility type definitions could also simplify working with that function. I can write them locally, but like. Someone else might have this problem I guess.

fn example_signature<T: Entity>(input: impl Into<PrimaryKey<T>>)

I can put in a PR if you're cool with this.

[neoeinstein]

I'm good with a PR for that. Thanks!