add: Support hex notation by 0xabcd when converting to bignum values

[BenBE]

As some if not most big numbers are usually given in hex it's nice to have a direct way of importing/converting them from string. Given that parsing hex to binary is even much faster than base conversion from base 10 to base 256 (for binary) it's highly recommended to prefer hex input for performance.

For output both formats have their advantages ...

[EvanCarroll]

Ohh, I see what asc2bn does,.

BN_asc2bn() infers the number base from an optional prefix. If str starts with “0x” or “0X”, it calls BN_hex2bn(), otherwise BN_dec2bn().

That's brilliant for coercion. Though I think we should keep the same behavior for in/out.