VAL-23 Implement rounding per 4626

[ams9198]

Updates the rounding per 4626 to round UP in certain cases. Per 4626,

• convertToAssets, convertToShares should both round DOWN
• previewDeposit, and deposit should round DOWN (i.e. calculating how many Y shares to mint for X assets)
• previewMint, and mint should round UP (i.e. calculating how many Y assets to transfer from lender to mint X shares)
• previewWithdraw, and withdraw rounds UP (i.e. calculating how many shares required to withdraw assets)
• previewRedeem, and redeem rounds DOWN (i.e. calculating how many assets to transfer TO lender for X shares)

Please sanity check that's correct: https://eips.ethereum.org/EIPS/eip-4626#security-considerations

Also simplified some of the PoolLib methods since they were duplicative at this point (consolidated to a single PoolLib.calculateConversion method).

[bricestacey]

Looks good -- confirmed with the EIP and your implementation.

The only part that was not explicit is withdraw and redeem. I believe the EIP doesn't mention how they should be rounded, but I assume it follows the preview methods.