Implement difficulty and performance calculations.

[naoei]

Supersedes #133.
Special thanks to @Coppertine @Flutterish, @YukiSnow0, Homura, and many others for helping.

Here's a quick breakdown of how we're calculating difficulty currently.

Preprocessing

Currently, we have two difficulty hit objects, the ordinary difficulty hit object (for hard beats) and an angled version (for beats, sliders, and slider repeats).

Regular difficulty hit object

This has nothing special attached to it, only providing a property called StrainTime with a minimum value of 25ms.

Angled difficulty hit object

This inherits the normal difficulty hit object and provides information around the current AngleRange, the Distance (or in other words, a delta angle) in angles between the current and last knowable angle, which if the previous hit object is a slider, it will fetch the very last angle of that slider.

If the current object is a Slider, then it will also provide information such as TravelDistance, LazyTravelDistance (travel distance - angle range), and the TravelTime.

Skills

We've combined the Star rating calculation and Performance points calculation into each skill, just to keep things simpler and easier to think of in mind.

Aim

The aim skill can be simply described by this mathematical operation:

$\dfrac{\texttt{distance}}{(\dfrac{\texttt{delta}^{1.4}-77}{100})^2 \cdot 0.1 + 25}$

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The rest of the skills we copied from osu! lol