Add ibc stargate to vm

[ethanfrey]

Addresses #692

• Add calls in cosmwasm_vm to execute code for all 6 ibc entry points.
• Add test helpers for these calls

[ethanfrey]

Rebasing #714 on top of this branch, so I can test it with real data. But I would like to merge these changes independent of the other contract

[ethanfrey]

@webmaster128 this should be good to merge, unless you have any change requests.

I tested these calls in #714
I can update them later if needed

[webmaster128]

Suggested change

	#[cfg(feature = "stargate")]
	fn default_features() -> HashSet<String> {
	features_from_csv("staking,stargate")
	}
	#[cfg(not(feature = "stargate"))]
	fn default_features() -> HashSet<String> {
	features_from_csv("staking")
	}
	fn default_features() -> HashSet<String> {
	#[cfg(feature = "stargate")]
	{
	features_from_csv("staking,stargate")
	}
	#[cfg(not(feature = "stargate"))]
	{
	features_from_csv("staking")
	}
	}

[webmaster128]

Not very pretty but avoids any mismatch in the function signature

[ethanfrey]

They will have the same compile-time checks, and same code.
If there is a mismatch it is only detected under one cfg option. (The CI will catch any compile-time issues)

I find the current version more readable