Rename `init` entry point to `instantiate`

[ethanfrey]

After seeing the handle -> execute rename, I started thinking of the other names. We still use init in CosmWasm, while we use Instantiate in wasmd. Further, the use of init as an export caused some issues in the TinyGo prototype (which I did manage to work around in the end), as init is a special function name in go (takes no args and is run on startup).

While we are doing renames, how about one more?

@webmaster128 your call if we squish this into the 0.14 release