Create package structure for stargate (Cosmos SDK 0.39) client lib

[webmaster128]

In order to provide a module and fully features client experience for Cosmos SDK 0.39 based blockchains, I suggest the following packages:

• @cosmjs/sdk39: Analogue to @cosmjs/sdk38. The high level CosmosClient with signing capabilities. Possible connectors: RestClient, TendermintClient, GrpcClient (optional, see https://docs.cosmos.network/master/architecture/adr-021-protobuf-query-encoding.html#grpc-protocol-support).
• @cosmjs/tendermint-rpc: for TendermintClient
• @cosmjs/proto-signing: Decorators allowing users to add support for custom types. Statically generated container types (maybe some messages for testing decorator implementations); everything for signing in direct mode

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• Rename @cosmjs/demo-protobuf to @cosmjs/proto-signing
• Create @cosmjs/tendermint-rpc (Fork @iov/tendermint-rpc #204)
• Create @cosmjs/sdk39

[webmaster128]

Thinking about this more, I wonder if we really need to have decorators separate. The light/minimal versions of protobufjs should only make a difference if you load the full library into the browser. But with any app optimizer as the one bundled in the React tooling, the unused symbols are stripped off anyways.

The advantage of using our decorators for standard and custom messages is:

• no static code generation needed for standard messages
• the same type/experience for standard and custom types

[webmaster128]

#241 shows how easy it is to create real world messages using decorators. This supports the idea of making decoratorsthe primary way to add message types, both internally and externally. I will update the original descriton to only use 3 packages instead of 4.