chore: rename instances of "delta" to "deltas"

[Jun1on]

Related Issue

BalanceDelta and BeforeSwapDelta are custom types of 2 int128s packed into a single 32 byte slot. However, other contexts like CurrencyDelta refer to a single value.

This singular naming scheme is confusing.

Description of changes

Rename BalanceDelta, BeforeSwapDelta, and other "delta" namings to plural form. Rename hook flags to "ReturnDeltas" except for afterSwap (which only returns one delta).