Reintroduce deprecated version from fromUnparsedString

As this instance method is being actively used by a project, I'm
reimplementing this method in the short term, marking it as deprecated,
which should usually introduce compiler warnings.

Sources/UInt128.swift

@@ -89,13 +89,17 @@ public struct UInt128 {
                   lowerBits: source.value.lowerBits)
     }
 
+    /// Initialize a UInt128 value from a string.
+    ///
+    /// - parameter source: the string that will be converted into a
+    ///   UInt128 value. Defaults to being analyzed as a base10 number,
+    ///   but can be prefixed with `0b` for base2, `0o` for base8
+    ///   or `0x` for base16.
     public init(_ source: String) throws {
-        if let result = UInt128._valueFromString(source) {
-            self = result
-        }
-        else {
+        guard let result = UInt128._valueFromString(source) else {
             throw UInt128Errors.invalidString
         }
+        self = result
     }
 }
 
@@ -744,6 +748,21 @@ extension UInt128 : ExpressibleByStringLiteral {
     }
 }
 
+// MARK: - Deprecated API
+
+extension UInt128 {
+    /// Initialize a UInt128 value from a string.
+    ///
+    /// - parameter source: the string that will be converted into a
+    ///   UInt128 value. Defaults to being analyzed as a base10 number,
+    ///   but can be prefixed with `0b` for base2, `0o` for base8
+    ///   or `0x` for base16.
+    @available(swift, deprecated: 3.2, renamed: "init(_:)")
+    public static func fromUnparsedString(_ source: String) throws -> UInt128 {
+        return try UInt128.init(source)
+    }
+}
+
 // MARK: - FloatingPoint Interworking
 
 extension FloatingPoint {

Tests/UInt128Tests/UInt128Tests.swift

@@ -1014,6 +1014,14 @@ class ExpressibleByStringLiteralTests : XCTestCase {
     }
 }
 
+@available(swift, deprecated: 3.2)
+class DeprecatedAPITests : XCTestCase {
+    func testFromUnparsedString() {
+        XCTAssertThrowsError(try UInt128.fromUnparsedString(""))
+        XCTAssertEqual(try UInt128.fromUnparsedString("1"), UInt128(1))
+    }
+}
+
 class FloatingPointInterworkingTests : XCTestCase {
     func testNonFailableInitializer() {
         var tests = [(input: UInt128(), output: Float(0))]