Octal and binary numbers > 2^63-1 crash the parser

[mroch]

we're using Int64.of_string to parse octal and binary numbers (e.g. 0o1234 or 0b10101). Int64.of_string throws when called with a number larger than Int64.max_int (2^63-1).

Instead, I think we can just use string_of_float. It seems to support 0b and 0o.