Expose _CurrencyImplementation as CurrencyProtocol

Motivation:

Working with currencies in generic cases aren't always practical with the type-erased `AnyCurrency` protocol, as repeated type constraints are added to access `Comparable` and `Hashable` conformances.

The `_CurrencyImplementation` doesn't need to be private, as anyone could conform to the protocol to gain much of the flexibility of `AnyCurrency` while also working in hashing contexts such as `Set`.

Modifications:

- Rename `_CurrencyImplementation` to `CurrencyProtocol` and make public
- Remove `CurrencyMetadata` superset requirements from `CurrencyProtocol`

Result:

Developers should have more flexibility with the Currency module types to work in existential or generic contexts

Sources/Currency/CurrencyProtocol.swift

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Currency open source project
+//
+// Copyright (c) 2020 Currency project authors
+// Licensed under MIT License
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Currency project authors
+//
+// SPDX-License-Identifier: MIT
+//
+//===----------------------------------------------------------------------===//
+
+/// Represents a type that acts as a currency.
+///
+/// Any `CurrencyProtocol` type behaves exactly like an `AnyCurrency` type, while also providing other capabilities such as
+/// `Comparable`, `Hashable`, etc.
+///
+/// This is the type to work with in most generic cases, as `AnyCurrency` is a type-erasure protocol when existentials are needed.
+public protocol CurrencyProtocol: AnyCurrency,
+  Comparable, Hashable,
+  ExpressibleByIntegerLiteral, ExpressibleByFloatLiteral,
+  AdditiveArithmetic
+{ }