feat: replace Multicall with our own MultiDelegateCall

[danielattilasimon]

Multicall is GPL'd, so we can't inherit it from MIT-licensed code.

[bingen]

Looks good, but are we planning to use this? What for?
If we don’t have clearly defined use case for this, I’d rather remove it. If we do, maybe we can add a test for it.

[danielattilasimon]

Turns out there is a test, though it's pretty nonsensical:

V2-gov/test/Governance.t.sol

Line 1412 in 078a766

function test_multicall() public {

Honestly, dunno why Multicall was included in the first place, maybe @ColinPlatt knows.