fuzz test pool library for solvency

[moodysalem]

this should be much easier to do now that pool logic is in a library. verify that via calls to modify position and swap, all positions will always be solvent. we should probably use mythx for this. tried it with forge, but it has some drawbacks over echidna/mythx style fuzz tests, mainly does not test sequences of transactions, but handling reverts is also painful
b18e172

[marktoda]

I have an idea for how to do this with forge - giving it a shot

[hensha256]

@marktoda to redo this with invariant tests