Deprecate safeMath operations by using Solidity compiler 0.8.0

[Ro5s]

The most recent implementation of Solidity lang (0.8.0) includes default under-over-flow checks for arithmetic operations. This therefore deprecates the use of the SafeMath library and improves code legibility ("+" will usually be more familiar than ".add()").

There can be increased gas costs however, such as the needless checks on for loops as noted by Nick Mudge.

I wanted to flag this potential improvement earlier on, as this could inform syntax of adapter development going forward.

[adridadou]

@craigwblake is looking into it