Expose hex charCode functionality

[ghost]

There's a clever abuse of ie-hex-str in the __encode-uri-char function used to get the hexadecimal representation of a character code. I would find this useful in my own functions. Can we break that into a separate function?

Generic number-to-hex would be nice, which could be composed with char code functions as needed.

[davidkpiano]

Sure - might take me a while to get to it though. If you want to get to it before me, feel free to open a PR!