fast string repeat

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Multiplies text `n` times. Optimized for almost any value of `n`.

The subroutine is intended to be embedded in flows, that's why it has extra safety measures, to save resources (space, time, and energy), and for algorithmic correctness.


## Why/how it works:

0. For low `n`, it uses a loop-less "SIMD-like hack" that takes advantage of mostly-documented "quirks":
0.0. `join` converts an array of `null`s into a sequence of "null" strings. This is undocumented behavior!
0.1. Using `join` saves more memory than `++`, because there's no ", " delimiter.
0.2. "(?s)" is the dot-all regex flag, it avoids conditional branching, increasing performance. We use "...." instead of "null" for the same reason (branchless computation).
0.3. `replaceAll` will iterate over non-overlapping chunks of 4 chars, replacing each one by our input string, returning the desired result.

1. For high `n`, it switches to shift-&-add multiplication algorithm: https://2ality.com/2014/01/efficient-string-repeat.html
1.0. The constant `0xffff` is arbitrary. I haven't done any benchmarks to confirm if it's an optimal threshold.

Tags: multiply pad padding string

LICENSE: https://unlicense.org

Rudxain profile: llamalab.com/automate/community/flows/42921