Add Genesis Siacoin Allocation

The Sia blockchain does not set a genesis Siacoin allocation. However, having the constants in place to support a genesis Siacoin allocation would make it easier for genesis forks of Sia to set up an airdrop. It would also simplify documentation on how to execute an airdrop when starting a genesis fork of Sia.

Additionally, the Genesis block should not be limited to having all Siacoin and Siafund outputs in the same transaction. Doing so causes confusion as there is no limitation in the types.constants.go file but there is elsewhere in the code. The end result is that without this change there is an undocumented constraint on how the Genesis block should be structured.