Angle type

I wonder if we might rather define our trig functions in terms of an `Angle` type:

```swift
public struct Angle {
public var radians: T
public init(radians: T) { self.radians = radians }
public static func radians(_ val: T) -> Angle { .init(radians: val) }

public var degrees: T { radians * 180 / .pi }
public init(degrees: T) { self.init(radians: degrees * .pi / 180) }
public static func degrees(_ val: T) -> Angle { .init(degrees: val) }
}
```

Browsing [SE-0246](https://github.com/apple/swift-evolution/blob/master/proposals/0246-mathable.md), this alternative design doesn't seem to have been considered, but I think it would help to make our functions more useful and self-documenting. Depending on what you're doing, it can be more natural to think in terms of degrees rather than radians. In C, people tend to bring these utility functions/macros by themselves, whereas other languages like Java [include them](https://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#toRadians(double)) as part of the standard library.

The design of the type given above means that creating and consuming an `Angle` in radians in essentially free.

Clearly, we cannot make this change to the `ElementaryFunctions` protocol requirements, as we need the conforming type to provide those implementations. However, we can do this for the free functions - either in addition to, or in place of the regular functions which take a `` argument.

Start by reviewing the ElementaryFunctions protocol and the free trigonometric functions referenced in the issue. Determine whether the proposed Angle type should supplement or replace the existing free-function interfaces; done means the design decision is resolved and the affected public API is implemented with coverage for radians and degrees.

Written by the indexing model from the issue text.