always decode Currency into new memory

Decoding into existing memory caused problems with global variables, because decoding into a global affects all other users of the global. This bug didn't rear its head until now because we very rarely decode into global variables. It also is not triggered when decoding into types.ZeroCurrency, since the internal slice is nil. In other words, it can only affect shared state when decoding modifies the previously-allocated memory of an internal slice.

Originally I thought we would need to need to resort to build.Critical here, but that turns out to be unnecessary. Allocating a new Currency for each decode works just fine.

There is one caveat, though, which is that you can still modify a global variable by decoding into it explicitly. For example, this code will change the global value of types.SiacoinPrecision:

fmt.Sscan("7", &types.SiacoinPrecision)

However, this code is quite obviously wrong, and not much different from writing something like types.SiacoinPrecision = types.ZeroCurrency. In both cases it is clear that the variable itself is being modified, so I think this is acceptable.