Increase max hexadecimal number

[guillemap]

Probably related to #135

The issue is, for big numbers the input field becomes 0, for example for a hex number like 1c7af1bbebe1d6222e59
This makes the tool unusable for example when decoding hex data from Ethereum transactions where numbers are often bigints with +18 digits.

Expected behavior:

         Hexadecimal                     Decimal
1c7af1bbebe1d6222e59 -> 134494183072681609539161

Current behavior:

         Hexadecimal                     Decimal
1c7af1bbebe1d6222e59 ->                        0
 c7af1bbebe1d6222e59 ->                        0
  7af1bbebe1d6222e59 ->                        0
   af1bbebe1d6222e59 ->                        0
    f1bbebe1d6222e59 ->                        0
     1bbebe1d6222e59 ->       124952769875553881
      bbebe1d6222e59 ->        52895175837625945