Migration path for math.IntProto to math.Int (breaking)

[damiannolan]

Handle migration of math.IntProto (proto encoding) to math.Int (binary encoding).

See thread in PR #7261.

Confirmed with @julienrbrt that this is breaking.

func TestMathIntIntProtoEqual(t *testing.T) {
	mathInt := math.NewInt(100)
	interfaceRegistry := testutil.CodecOptions{}.NewInterfaceRegistry()
	codec := codec.NewProtoCodec(interfaceRegistry)

	ip := types.IntProto{Int: mathInt}
	ipBz := codec.MustMarshal(&ip)

	bz, err := mathInt.Marshal()
	require.NoError(t, err)
	require.Equal(t, ipBz, bz)

	amount := math.Int{}
	err = amount.Unmarshal(ipBz)
	require.NoError(t, err)
	require.Equal(t, mathInt, amount)
}

The above fails.

The IntProto type will be readded to sdk and the recommendation is to do a migration. Either with upgrade handler or lazy migration by updating this k/v entry to use collections.

Originally posted by @damiannolan in #7261 (comment)

[julienrbrt]

You should be able to use the AltValueCodec from collections: https://github.com/cosmos/cosmos-sdk/blob/collections/v1.0.0-rc.1/collections/codec/alternative_value.go#L20

[damiannolan]

Thanks, will try to take a look at it in the coming days

[tac0turtle]

the code that changed this can be reverted, we brought it back. it should save a migration and extra work. happy to knock it out if needed