Add a `clamp` function to Algorithm.swift

[kylemacomber]

I like the idea of a median free function a lot:

• It's generally useful beyond just clamping.
• It seems like a natural, missing peer to min and max.
• It neatly avoids the question of if/how to handle half-open ranges.
• For me, a free function is a more intuitive way to surface this functionality (struggling to articulate why).