SE-0329 defines the following static factory methods:
```
public static func seconds<T: BinaryInteger>(_ seconds: T) -> Duration
public static func seconds(_ seconds: Double) -> Duration
public static func milliseconds<T: BinaryInteger>(_ milliseconds: T) -> Duration
public static func milliseconds(_ milliseconds: Double) -> Duration
public static func microseconds<T: BinaryInteger>(_ microseconds: T) -> Duration
public static func microseconds(_ microseconds: Double) -> Duration
public static func nanoseconds<T: BinaryInteger>(_ value: T) -> Duration
```
For no good reason, the obvious additional method:
```
public static func nanoseconds(_ nanoseconds: Double) -> Duration
```
was omitted. After talking this through with the LSG, we have decided
that this is simply a bug, and we will add this method without formal
evolution review.
The numerics of these operations is fairly subtle, and the overflow boundaries are somewhat surprising, so additional test coverage isn't a bad thing (also gives us an excuse to exercise Int128 more)
* Fix sign of fractional units in Duration initializers from Double.
When separating a double duration into integral and fractional parts, we got the sign wrong. This fixes that bug.
* Fixup variable name.
When constructing a Duration from Double, we should do it in such a way that exact integer inputs are preserved exactly, so long as they are represented as a Duration. This was not previously the case. Now it is.
