Add Gray Code holes

[jakobkordez]

Gray code decoder:
For each argument in reflected binary (Gray code) form, print the same number in decimal form.

Gray code encoder:
For each numeric argument in decimal form, print the same number in reflected binary (Gray code) form.

The numbers range from 0 to 4096 inclusive.

[hallvabo]

The encoder is too trivial. Maybe more interesting if encoder/decoder is merged into one hole, and direction decided by whether the input number is in binary or decimal?

[MichalMarsalek]

This shall now be the issue for the Decoder, so that the two can be voted for independently.
Encoder: #1157