Store currency/security values/prices using big.Rat natively

This adds 'shadow' types used only by the store/db internal package whch
handle converting these types to their DB-equivalent values. This change
should allow reports to be generated significantly faster since it
allows a large portion of the computation to be shifted to the database
engines.

internal/models/prices.go

@@ -12,7 +12,7 @@ type Price struct {
 	SecurityId int64
 	CurrencyId int64
 	Date       time.Time
-	Value      string // String representation of decimal price of Security in Currency units, suitable for passing to big.Rat.SetString()
+	Value      Amount // price of Security in Currency units
 	RemoteId   string // unique ID from source, for detecting duplicates
 }