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.
Hmm. I'm not generally a big fan of types like `tick_t` or `hash_t` or `equals_t` or anything like that - and changing it would break code, so we'd be forced to go through a deprecation path anyway. So, we might as well just go with `ticks` as a `long` and then go through the appropriate deprecation path later if we have to.

But the only reasons that I can think of why we would _need_ to change from `long` would be if another OS used a type that could hold more than a `long` can hold, and it didn't start at 0 - or if a system had precision in the realm of picoseconds and could give larger values than a `long` can hold. Anything else could just fit in the `long`. All of the stuff that I was doing before that was going to require holding the time with the seconds split out was to cover the whole range from POSIX, which we decided wasn't necessary, because the range was too large to matter, and it will be the same for any system unless it provides a type whose range of values exceeds what a `long` can hold, and its clock starts high enough above 0 that we risk not being able to hold the value in a `long` during normal program execution.

So, I don't think that we'll need to change the internal representation, but if we do, I'd prefer to just go through the normal deprecation cycle to replace `ticks`. Walter frowns enough on breaking code that I doubt that it would fly to even mark a function to indicate that it wasn't guaranteed to stay the same across releases.