StringRankedSearch inconsistent results when numbers and letters are mixed

Demonstrated by the following unit tests (originally reported here https://github.com/wordpress-mobile/WordPress-iOS/pull/23776/):

  func testSearchBeginingOrEnd() {
         XCTAssertEqual(score("AB00", "AB"), score("ABXX", "AB"), accuracy: 0.1)
         XCTAssertEqual(score("4200", "42"), score("42XX", "42"), accuracy: 0.1)

         XCTAssertEqual(score("00AB", "AB"), score("XXAB", "AB"), accuracy: 0.1)
         XCTAssertEqual(score("0042", "42"), score("XX42", "42"), accuracy: 0.1)

         XCTAssertEqual(score("AB_00", "AB"), score("AB_XX", "AB"), accuracy: 0.1)
         XCTAssertEqual(score("42_00", "42"), score("42_XX", "42"), accuracy: 0.1)

         XCTAssertEqual(score("00_AB", "AB"), score("XX_AB", "AB"), accuracy: 0.1)
         XCTAssertEqual(score("00_42", "42"), score("XX_42", "42"), accuracy: 0.1)

         XCTAssertEqual(score("AB/00", "AB"), score("AB/XX", "AB"), accuracy: 0.1)
         XCTAssertEqual(score("42/00", "42"), score("42/XX", "42"), accuracy: 0.1)

         XCTAssertEqual(score("00/AB", "AB"), score("XX/AB", "AB"), accuracy: 0.1)
         XCTAssertEqual(score("00/42", "42"), score("XX/42", "42"), accuracy: 0.1)
     }

     func testCompareSearchNumbersAndLetters() {
         XCTAssertEqual(score("42XX", "42"), score("ABXX", "AB"), accuracy: 0.1)
         XCTAssertEqual(score("0042", "42"), score("XXAB", "AB"), accuracy: 0.1)
         XCTAssertEqual(score("42_00", "42"), score("AB_XX", "AB"), accuracy: 0.1)
         XCTAssertEqual(score("00_42", "42"), score("XX_AB", "AB"), accuracy: 0.1)
         XCTAssertEqual(score("42/00", "42"), score("AB/XX", "AB"), accuracy: 0.1)
         XCTAssertEqual(score("00/42", "42"), score("XX/AB", "AB"), accuracy: 0.1)
     }

Start by locating StringRankedSearch and the unit-test target containing the score(...) helper. Use the demonstrated assertions as regression criteria for mixed numbers and letters, including separators and matches at either end. Done means all listed comparisons pass with the stated accuracy.