These operations implicitly truncate their parameters, and result to
integers:

* not
* and
* or
* xor
* shl
* shr
* mod
* trunc

Base 2 was left out of the base conversion code intentionally as it
would require making the UI at least one third wider.

Attempts to change base with negative values will simply display
"error." Note that with larger numbers, the result may be inaccurate
due to rounding.

I've also bound the Return key to the equal() action.

Signed-off-by: Tim Hentenaar <tim@hentenaar.com>