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

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`.