Optimize animation import

[ashikns]

Improves upon three things:

1. Do tangent calculation operations on keyframe cached array rather than AnimationCurve. This is the most important change. Unity profiler shows that accessing AnimationCurve.keys is a costly operation.
2. Optimizes branching by moving if..else outside loop. The compiler probably already does this but it's nice to do it ourselves.
3. Fixes a bug when two incoming animation keys have the same time value. I don't know if this is valid per gltf specs but I faced this using some models downloaded from Sketchfab.

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[ashikns]

Just saw that there's another PR that includes first point from this: #479

[blgrossMS]

Thank you for contributing the change! @AdamMitchell-ms can you review?