Add a lint to check for functions/methods that could be From

What it does

Checks for functions and methods that could be a From impl (we could even do this for TryFrom)

Advantage

• Makes the code more idiomatic

Drawbacks

No response

Example

struct StructA {
    some_field: String
}

struct StructB {
    some_field: String
}

fn map_to_struct_b(a: StructA) -> StructB {
    StructB {
        some_field: a.some_field
    }
}

impl StructA {
    fn to_struct_b(self) -> StructB {
        StructB {
            some_field: self.some_field
        }
    }
}

Could be written as:

impl From<StructA> for StructB {
    fn from(a: StructA) -> Self {
        StructB {
            some_field: a.some_field
        }
    }
}

Start from the function and method examples in the issue, then review existing Clippy lint conventions and tests for how new lints are structured. Define the qualifying conversion cases and exclusions before implementation; done means the lint reliably identifies the intended cases and has coverage for the examples and relevant edge cases.