Reorganise and test std::time

std::time is now broken up into two modules:

1. std::time, which provides time related types.
2. std::time::duration, which provides the Duration type.

This separation allows for the use of module methods to construct a
Duration using different time units, instead of using methods defined on
Duration itself. This means that instead of this:

    import std::time::Duration

    Duration.from_seconds(5)

You now write this:

    import std::time::duration

    duration.from_seconds(5)

== Renaming MonotonicTime to Instant

MonotonicTime has been renamed to "Instant", which is a bit shorter and
much easier to type. A new method has been added as well: "elapsed".
This method can be used to measure the time that has elapsed since the
Instant was created.

== Adding and subtracting SystemTime and Instant

The methods + and - for SystemTime and Instant no longer accept a
ToFloat, instead they require a Duration. Accepting a ToFloat would
allow you to add a SystemTime to another SystemTime, or add an Instant
to an Instant. Neither make sense, so instead we now require the use of
a Duration. This means that instead of this:

    import std::time

    time.now + 5

You now have to write this:

    import std::time
    import std::time::duration

    time.now + duration.from_seconds(5)

== Tests

Finally, both std::time and std::time::duration have tests. These tests
uncovered a few bugs in the implementation of SystemTime, which have
been resolved.
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