Make storage a namespace to prevent allocation

[willemneal]

storage class had no internal state so allocation wasted space and gas.

Also added assemblyscript types reference in types file.

[vgrichina]

@willemneal the allocation of one object doesn't really add any meaningful overhead.

I'd prefer we keep this as class as it's then easier to switch to a model where you can inject any Storage object (including fake one for tests), though it requires abstract methods support landing in AS.

[willemneal]

I think that the best way to mock the storage is at the API level. And while it is minimal overhead, it's still extra gas for every contract that uses this.