Cache wasmer_runtime_core::Instance instead of cosmwasm_vm::Instance

[webmaster128]

Caching the lower level wasmer_runtime_core::Instance would allow us to remove the public function cosmwasm_vm::Instance.set_gas and instead pass the limit at construct time (as shown in #136).

This would require the ability to preserve cosmwasm_vm::Instance.wasmer_instance when the container Instance is destructed in store_instance, just like storage.

Postponing this for now as not urgent.

[webmaster128]

Is this helpful? https://doc.rust-lang.org/std/mem/struct.ManuallyDrop.html

ManuallyDrop::take is stable from Rust 1.42.0 on.

[webmaster128]

Done in #140 on branch 0.7

[ethanfrey]

We can close this as well. All this cleanup is 0.7 stuff