Add math.length()

[wxsBSD]

Add a math.length() which will return the length of the sequence of bytes, including any NULL bytes.

Fixes #1778.

[plusvic]

I find a bit weird that this is included in the math module. Maybe it's time for a string module?

[wxsBSD]

Sure, I'll do it today.

[wxsBSD]

I decided to move the to_int() functions from the math module over to the new string module as it made more sense to me to convert to an integer via string.to_int() instead of math.to_int(). If you want to convert the other way around you still use math.to_string().

[wxsBSD]

Having things in a string module is a nice idea, it opens up the possibility of substring functions and other related things.

[wxsBSD]

Merged in the next branch.