Integrate warm instances

[camilbancioiu]

This PR fully enables warm instances, VM-wide.

Warm instances are a lower-level of caching. WASM contracts, once instantiated, are kept in memory for future calls, ready to execute. This saves both compilation and instantiation time.

The speed-up potential is immense: once instantiated, contracts are always executed natively on the CPU because they are not WASM anymore.

This optimization is specific to the VM only and needs no support from the node.