`isAlNum` and `isAlpha`:
1. I think these names are a bit cryptic.
2. `isAlpha` is a bit ambiguous: is it alpha*numeric* or alpha*betic*?
   This is why I renamed `isAlpha` to `isAlphabetic`.
3. For consistency and because `isAlNum` looks weird, I renamed it to `isAlphanumeric`.

`isCntrl`:
1. It's cryptic and hard to find when you look for it.
2. We don't save a lot of space writing it this way.
3. It's closer to the name of the `control_code` struct.

`isSpace`:
1. The name is ambiguous and misleading.

`spaces`:
1. Ditto

`isXDigit`:
1. The name is extremely cryptic.
2. The function is very hard to find by its name.