From srml/balances module:

Somewhat ugly hack in order to gain access to module's increase_total_issuance_by
using only the Subtrait (which defines only the types that are not dependent
on Positive/NegativeImbalance). Subtrait must be used otherwise we end up with a
circular dependency with Trait having some types be dependent on PositiveImbalance
and PositiveImbalance itself depending back on Trait for its Drop impl (and thus
its type declaration).

This works as long as increase_total_issuance_by doesn't use the Imbalance
types (basically for charging fees).

This should eventually be refactored so that the three type items that do
depend on the Imbalance type (TransactionPayment, TransferPayment, DustRemoval)
are placed in their own SRML module.