enhancement: import open zepplin or solmate properly and make upgrade to 0.8

[Ro5s]

Currently MISO token launchpad uses older versions of OpenZeppelin helper and token contracts, compiler hardcoded at 0.6.12, stored locally here.

It might make more sense to import these contracts from OZ codebase (so we track upgrades, simplify MISO repo, use >=0.8.0 solidity), or in the alternative, explore using Solmate templates. which are simpler, more opinionated and gas-optimized.

[matthewlilley]

@Ro5s just said earlier that we should import OZ properly, but would be keen to see an update using Solmate & 0.8. You keen?