Extract loanRatesFor() to replace the identical 4-line
managementFeeRate/interestRate/periodicRate preamble repeated in
makeFullPayment, makeLatePayment, and makeRegularPayment.
Clarify the constructLoanState(SLE::const_ref) doc comment to note
it's an overload of the 3-Number constructLoanState() above it,
rather than reading as a standalone function.