Add a built-in `POPCOUNT` function

Eventually:tm: we'll have user-defined functions (#201), and people can implement all kinds of inline numeric logic -- even `POPCOUNT`, assuming they support recursion and a short-circuiting ternary operator:
```asm
DEF POPCOUNT(x) := x ? (1 + POPCOUNT(x & (x - 1))) : 0
```

However, that's not an immediately obvious definition, and we won't have user-defined functions for a long while yet, so it would be helpful to provide `POPCOUNT` specifically as a built-in. Unlike some desired functions, you can't just "fake it" with a single expression (e.g. `DEF tiles EQUS "* 16"` instead of `DEF tiles(x) := x * 16`); you have to use a `REPT`/`FOR` loop.

I'm bringing this up again because someone in pret was figuring out a `mulhl` macro (an instruction macro to optimally "multiply `hl` by argument"; yes I know the arguments against such things, but when users are going to write them anyway, rgbasm shouldn't make them even *more* verbose).

`POPCOUNT` will make sense along with our `BITWIDTH` and `TZCOUNT` functions.

Start by locating the existing BITWIDTH and TZCOUNT built-in functions and their tests, since POPCOUNT is intended to fit alongside them. Add the built-in with the recursive behavior described in the issue, then verify its results across representative numeric inputs using the existing test structure.

Written by the indexing model from the issue text.