Negative nos support for dec to hex and hex to dec conversion

[murtraja]

Hello,

If you try to convert a decimal negative no. to hex, the program crashes, or if you've previously done some calculations, it shows a random hex value like 28B6718E

I tried to debug this issue, by forking my copy and running make and then running gdb src/gnusim8085 with breakpoint set in the conv_dec_to_hex function. I noticed that the argument to the function is 0 if you've given a negative number. (tried p dec command)

I would like to work on adding support for negative nos conversion by considering the no. to be just 8 bits. We can have a checkbox which toggles this behaviour