Add number, string and array arithmetic

Addition (+), subtraction (-), multiplication (*) and division (/) for numbers
follows the BIDMAS rules.

Strings and arrays can be concatenated with the addition operator

Strings can be concatenated with numbers with the addition operator
Add string tr() function

tr() takes two string arguments, translating each character of the string in
argument 1 to the corresponding character in argument 2. Character classes
([a-z]) are not supported at this time.