Rename package name to be hyphenated to match idioms

Motivation:

Across the ecosystem it has become an idiom to have the Package name match the repository name. SwiftPM even assumes as such in several cases.

The current name conflicts with this expected idiom.

Modifications:

- Rename: Package from `Currency` to `swift-currency`

Result:

The package should match expected ecosystem idioms for naming.

Package.swift

@@ -4,7 +4,7 @@
 import PackageDescription
 
 let package = Package(
-    name: "Currency",
+    name: "swift-currency",
     products: [
         .library(name: "Currency", targets: ["Currency"]),
     ],