Better tracking of CosmWasm capabilities

[alpe]

They are currently defined in a string in app.go. Updates can be missed easily as the file is quite big already.

Some ideas:

• extract to a dedicated method that returns []string, format in app.go with strings.Join
• new file in /app that contains the new method

[webmaster128]

Using a string array (or better set if you have that type available) is the right thing to do here. The only reason this is a comma separated list in wasmvm is the FFI boundary to Rust. We should have internalized this string representation from the beginning.

CosmWasm/wasmvm#425