Make instantiate export optional

[webmaster128]

At some point there might be contracts that don't need an instantiate entry point. At the moment, calling Instantiate in wasmd requires it. But other callers or future wasmds can be more flexible. To prepare for this, we should remove the requirement for instantitate from the static validation. This changes some error cases from the StoreCode step to the Instantiate step, but this is not super bad.

Once it is optional, we should have a dev contract without the endpoint and create a test that shows how Instantiate in wasmvm behaves when the entry point is missing.

[chipshort]

One problem I encountered here: cosmwasm-schema currently requires an instantiate message and changing it is technically a breaking change because Api is public. Not sure if anyone is using it directly though.
I can create the contract with an Empty instantiate msg in the schema or without schema for now and we can make it optional in cosmwasm-schema 2.0

[webmaster128]

Okay great. In this case let's push the whole ticket into 2.0. It's not urgent enough to justify headaches.