feat(examples): add p/demo/seqid

[thehowl]

A very simple ID generation package, designed to be used in combination with avl.Trees to push values in order.

The name was originally seqid (sequential IDs), but after saying it a few times I realised it was close to "squid" and probably would be more fun if I named it that way ;)

There's another piece of functionality that I want to add, which is a way to create simple base32-encoded IDs. This depends on #1290. These would also guarantee alphabetical ordering, so a list of them can be easily sorted and you'd get it in the same order they were created. They would likely be 13 characters long, but I'm also thinking of making a compact version which works from [0,2^35) which is 7 chracters, and then smoothly transitions over to the 13 characters version when the ID is reached.

(I've experience with both base64 and base32 encoded IDs as 64-bit numbers, as I used both systems. The advantage of base32 is that it makes IDs case insensitive, all the while being at most 13 bytes instead of 11 for base64.)

In GnoChess, we used simple sequential IDs combined with zeroPad9 to create IDs which were both readable and sortable. I want to make a more "canonical" solution to this which does not have a upper limit at 1 billion entries.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7d66398) 55.94% compared to head (a4511f9) 47.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1378      +/-   ##
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- Coverage   55.94%   47.17%   -8.78%     
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  Files         420      372      -48     
  Lines       65415    61720    -3695     
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- Hits        36596    29115    -7481     
- Misses      25961    30220    +4259     
+ Partials     2858     2385     -473     

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