[swift-evolution] Keyword for protocol conformance

[David Cordero]

*The problem:*
At the moment, looking at the code of a class or a struct implementing a
protocol, it is hard to know what methods are actually implementing the
protocol and what other methods are just part of the code of the class or
struct.

People are trying to fix this problem with pragma marks or moving the code
conforming the protocol to a separate extension, but they are just specific
good practices or code styles.

*Proposal:*
Adding a keyword to the methods conforming protocols. As an example please
check the following piece of code which uses the keyword `conform` to
explicitly indicate that `myMethod` is a method conforming a protocol.

```
protocol MyProtocol {
    func myMethod() -> String
}

class MyClass: MyProtocol {

    *conform* func myMethod() -> String {
        return "Yuhuuu,I am conforming \\o//"
    }

    func whatever() {
        print("I am a boring method and I don't conform anything")
    }
}
```

It would be something similar to the current keyword `override` but for
protocols.

Apart from improving code readability, It would allow the detection, in
compilation time, of errors due to code evolution. For example redundant
methods that no longer conform anything once the requirement is removed
from the protocol for whatever reason.

Looking forward your opinions and comments.

Kind Regards
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