Avoid overflow and underflow of `length` and `distance`

• Replace x * x + y * y with hypot(x, y) (9feb2ea)
  • This prevents overflow and underflow during calculating length and distance. It makes it possible to pass the following tests.

XCTAssertEqual(CGPoint(x: .greatestFiniteMagnitude, y: 0.0).length, .greatestFiniteMagnitude)
XCTAssertEqual(CGPoint(x: 0.0, y: .leastNonzeroMagnitude).length, .leastNonzeroMagnitude)