Merge pull request #7870 from stephentyrone/FloatingPoint-Hashable

FloatingPoint should imply Hashable. 

 `Hashable` is related to `Equatable`, and `FloatingPoint` is `Equatable`, with enough additional constraints that `Hashable` conformance is practical.  

There's *almost* no excuse for any `Equatable` thing not to also be `Hashable`; the exception is when the underlying representation may be denormalized in some way that makes it impossible or expensive to normalize (e.g. `Set`).  Floating point numbers have denormalized forms, so this comes down to the cost of normalization.  Since we can't imagine a representation whose normalization is much more expensive than the subsequent hashing step, the conformance belongs.

Author: Dave Abrahams

stdlib/public/core/FloatingPoint.swift.gyb

@@ -168,7 +168,7 @@ word_bits = int(CMAKE_SIZEOF_VOID_P) * 8
 ///           "out of \(tempsFahrenheit.count) observations.")
 ///     // Prints "Average: 74.84°F in 5 out of 7 observations."
 public protocol FloatingPoint: Comparable, Arithmetic,
-  SignedNumber, Strideable {
+  SignedNumber, Strideable, Hashable {
 
   /// A type that represents any written exponent.
   associatedtype Exponent: SignedInteger