[Performance] Audit SortedDictionary insert(_:key:) performance

[jsbean]

Inserting:

• 1000 random (key, value) pairs takes 0.004 s
• 10,000 random (key, value) pairs takes 0.131 s
• 20,000 random (key, value) pairs takes 0.432 s
• 30,000 random (key, value) pairs takes 0.966 s
• 40,000 random (key, value) pairs takes 1.707 s

This doesn't seem like O(n) :o.

[jsbean]

This is the only thing getting called:

Actually, it looks like SortedArray is doing the same thing.

[jsbean]

This is dependent on: #164.