Add WasmMsg::Migrate variant

[ethanfrey]

Closes #761

After merging, this needs to be exposed in wasmvm and handled in wasmd

@ebuchman thanks for the suggestion

[webmaster128]

This is by the way a breaking change as the enum is not #[non_exhaustive]. If we ever want to add new cases in 1.x, we should make it #[non_exhaustive].

[ethanfrey]

You mean WasmMsg? (Which I think is pretty final, we are just finally supporting a feature we added in 0.9).

Or CosmosMsg in general, which definitely will expand sometime. I guess I could just add this to all the types here

[webmaster128]

This WasmMsg just brought up the topic. I mean CosmosMsg and all its sub-messages. It is just too easy to forget one type, or require a v2 of an existing type later on. If we need to make a CosmWasm 2.0, just because we did not write #[non_exhaustive] and use fallback cases in the match, it would be pretty annoying. I guess we do not even match those as they are decoded in Go directly from JSON.

[ethanfrey]

Huh, I made them non_exhaustive in two commits, and assumed some code would break, at such as places like

cosmwasm/packages/std/src/mock.rs

Lines 301 to 314 in 448671c

	match &request {
	QueryRequest::Bank(bank_query) => self.bank.query(bank_query),
	QueryRequest::Custom(custom_query) => (*self.custom_handler)(custom_query),
	QueryRequest::Staking(staking_query) => self.staking.query(staking_query),
	QueryRequest::Wasm(msg) => self.wasm.query(msg),
	#[cfg(feature = "stargate")]
	QueryRequest::Stargate { .. } => SystemResult::Err(SystemError::UnsupportedRequest {
	kind: "Stargate".to_string(),
	}),
	#[cfg(feature = "stargate")]
	QueryRequest::Ibc(_) => SystemResult::Err(SystemError::UnsupportedRequest {
	kind: "Ibc".to_string(),
	}),
	}

but tests keep working. Not sure what this does in the end

[webmaster128]

This is a surprise I also hit recently. Rust is too clever: crate-internal you always must mach all cases. Clippy also marks fallback as dead code when all cases are matched. Crate-external, you always need a fallback case.