Implement Api methods in cosmwasm_std::ExternalApi

[ethanfrey]

Part of #751
Requires #756

• Add imports to cosmwasm_vm::Instance for all the calls in cosmwasm_std::Api trait
• Use same implementation as in MockApi to implement these calls in the vm (native code)
• Add hooks in cosmwasm_std::ExternalApi to call these new entry points
• Add integration tests to the sample crypto app. Ensure these calls work full stack (request -> wasm, wasm -> vm -> wasm for verification, wasm -> response)

[webmaster128]

It turned out to be more helpful to work on the whole stack at once, and split the work algorithm by algorithm. As a consequence, this will always be done automatically for each method.