Consistent format of asset amount

[mrq1911]

getBestSell(in, out, amount).toTx(limit) is the worst offender, where the amount is expected to be normalized (123.33), byt limit is absolute (123330000) which leads to errors in integrations which can lead to loss of funds for users

getBestSpotPrice returns the amount of 1 asset in as {amount, decimals}, so you are forced to convert it youself even more confusingly amount can have the fractional part as well

proposed fix

introduce new type Amount that can be created automatically from Number, String, whatever and implements toString, toHex, toNumber, toBigNumber, decimals and toNormalized (name TBD) that will normalize asset amount by decimals, others will return absolute amount, alternatively, we could always deal with the normalized amounts and there would be toAbsolute

it is strongly recommended to to these changes in a back-compatible way to minimize errors this will introduce in integrations