Idea: Add link to the actual implementation of the opcode as described in the execution specs

[shafu0x]

For each opcode add a link to its implementation. For example all the Arithmetic opcodes implementation can be found here:

https://github.com/ethereum/execution-specs/blob/master/src/ethereum/byzantium/vm/instructions/arithmetic.py

[dorlevi]

sorry for the delay @SharifElfouly, that is a good suggestion, We do try to avoid linking to external resources that might change but in this case, it's pretty safe.