Converts decimal, hex or binary string representation into number.
DESCRIPTION
The call num_of_string "n" converts the string "n" into an OCaml
unlimited-precision number (type num). The string may be simply a sequence of
decimal digits (e.g. "123"), or a hexadecimal representation starting with
0x as in C (e.g. "0xFF"), or a binary number starting with 0b (e.g.
"0b101").
FAILURE CONDITIONS
Fails unless the string is a valid representation of one of these forms.