The go vet command complains about untagged struct initializers when
defining a ShaHash directly. This seems to be a limitation where go vet
does not exclude the warning for types which are a constant size byte
array like it does for normal constant size byte array definition.
This commit simply modifies the code to use a constant definition cast to
a ShaHash to overcome the limitation of go vet.
The genesis block for each network is a parameter for the network. As
such, it makes more sense to define them in this package instead of in the
wire protocol package.
ok @jrick
