Add MonoTime as a replacement for TickDuration

`TickDuration` conflates a timestamp of the monotonic clock and a duration in ticks of the monotonic clock, which is causing problems and confusion. In addition, it doesn't follow the same API as Duration (since it was developed separately by SHOO), even using the same function names for different things (e.g. `Duration.msecs` gives the milliseconds split out whereas `TickDuration.msecs` gives the total in miliseconds). And IMHO, having two duration types is overly confusing anyway. By having only one duration type, it's then quite clear which duration type code should use, whereas if they're are multiple, folks then have to understand the differences between them in order to figure out which to use where.

So, I'm proposing that we add `MonoTime`. It will represent a timestamp of the monotonic clock and will have almost no functionality on it (since it doesn't usually make sense to do much directly with a timestamp of the monotontic clock). It can be compared and subtracted but not much else. When subtracting two `MonoTime`s, you'll get a `Duration`. That does result in some rounding errors (because the clock frequency is being converted to hecto-nanoseconds) and might result in lower precision (though many systems don't have a clock resolution/frequency as a high as hecto-nanoseconds anyway), but it's eminently usable and user-friendly., and most code doesn't need precision higher than hecto-nanoseconds anyway.

    immutable before = MonoTime.currTime;
    // do stuff
    immutable after = MonoTime.currTime;
    auto timeElapsed = after - before;

For those rare cases where someone needs to actually operate in system ticks, they can use the `ticks` property of a `MonoTime` and its `ticksPerSecond` property to operate in ticks like `TickDuration` would do now. But most folks shouldn't need to do that.

This should simplify things considerably. The one area that might annoy people is that `Duration` has no support for floating point calculations (whereas `TickDuration` does), but in most cases, it's a bad idea to use floating point values when dealing with time, so in most cases, people probably shouldn't be using floating point anyway. But that can be discussed separately as an enhancement to `Duration` if it's deemed appropriate. `MonoTime` itself doesn't care.

For now, `TickDuration` has not been deprecated, because it's still used by the benchmarking functions in std.datetime. However, when I split std.datetime (which should be happening soon), I intend to create versions of those functions which use `Duration`, and put them in std.benchmark, where they're supposed to eventually end up anyway (but haven't due to issues with implementing std.benchmark). The old versions of those functions which use `TickDuration`, along with `TickDuration`, itself can then be deprecated.

I have however removed the few uses of `TickDuration` in druntime outside of core.time with this pull.
It's been a while since I wrote this, but I believe that the idea was that the values wouldn't overflow as quickly this way. It converts the seconds separately from the fractional seconds so that the resultant number isn't as large in the middle of the conversion. For instance, with the frequencies `10_000_000` and `1_000_000` and the value `long.max >> 20` for `ticks`, the conversion from `1_000_000` to `10_000_000` overflows and ends up with a negative value with your equation (and `long.max >> 19` overflows in both directions), whereas with my equation, it's actually able to avoid overflow with values as large as `long.max >> 1`.

And actually, it seems that my equation avoids overflow better than using a floating point conversion would (I had assumed that they would be comparable but that the floating point would have trouble with not being able to represent some numbers, making it less desirable). By having

    return cast(long)(cast(real)(ticks * dstTicksPerSecond) / srcTicksPerSecond)

it seems to be identical to your version (i.e. the cast to `real` has no real effect), whereas

    return cast(long)(ticks / (cast(real)srcTicksPerSecond) * dstTicksPerSecond);

is able to deal with much higher numbers, but it still chokes somewhat sooner than my equation (e.g. `long.max >> 4` is fine, but `long.max >> 3` is too large and overflows).

I actually tried to find somewhere online that discussed the best way to do this conversion, but I wasn't able to find anything (e.g. rounding would arguably improve the accuracy in some circumstances, but I don't know if that's really better or not). This equation was the best that I was able to come up with. And as far as I can tell, it's more accurate, because it's able to deal with larger numbers.