Since these constants can be useful for int64, Amount, and float64
math, it doesn't make sense to make them just one type, and require
type conversions for the rest.
ok @davecgh
Amount should still be a usable type even if the monetary amount being
described is not an amount at a single instance in time, for example,
the total of all BTC received by an address. Therefore, the bounds
checks that the amount is within the total amount of bitcoin ever
producable have been removed.
The checks for NaN and +-Infinity remain.
Besides being shorter, using "BTC" rather than "Bitcoin" in the
AmountUnit constants is deemed to be better for these units as BTC is
already a recognized monetary unit.
AmountBaseBitcoin has likewise been renamed to AmountSatoshi as this
is consistant with how it is returned as a string. The "standard"
part of the comment in the const block has been removed, as Satoshi is
technically not a standard term for this unit.
ok @davecgh
