Move vault share/asset conversion functions (assetsToSharesDeposit,
sharesToAssetsDeposit, assetsToSharesWithdraw, sharesToAssetsWithdraw)
and isVaultInsolvent from View.h/View.cpp into a dedicated
VaultHelpers.h/VaultHelpers.cpp module. Reorder includes in Vault
transactor .cpp files to place own header first. Fix VaultSet flag
validation logic.