A demo of Union/Find algorithm in ATS

[Steinway Wu]

Hi, 

I was reading a chapter about type inference and constraint solving in ML. It mentioned a quick Union/Find algorithm for testing equivalent classes. I translate the code into ATS, and pasted here at [glot.io: Union/Find](https://glot.io/snippets/eiaoq3icf1)

Just for fun.

-- 
Steinway Wu
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[gmhwxi]

I suggest:

fun{} addr_of{a:t@ype} (pt a): ptr

Casting a pointer(64bit) to an integer(32bit) is a bit dangerous.

[Steinway Wu]

Updated, thanks!