Improved Chain.build() performance by 1/3

[egormanga]

• Using dict.setdefault() is much faster than doing a separate dict.__contains__() check each iteration. This gives us ~1/6× better performance.
• But keeping the in-place addition and falling back by catching KeyError gives another 1/6 over using dict.get().

[S0yKaf]

This should be 1

[egormanga]

You're perfectly right, thank you :)

[S0yKaf]

Hopefully this gets merged, it's a good change! I've been using your branch because of it.