Keep in a global cache the Wasm modules obtained by compiling a given bytecode

[ma2bd]

It seems that we're doing too much redundant (and concurrent) compilation. This might be the reason why we hit #633 while other Wasmer/singlepass users don't.

Possible definition for WasmApplication:

enum WasmApplication {
    Wasmer { contract_module: wasmer::Module, service_module: wasmer::Module },
    Wasmtime { contract_module: wasmtime::Module, service_module: wasmtime::Module },
}

For now, we should probably only allow one compilation tasks of a new bytecode at a given time. (I.e. I would just hold the module compilation cache under a lazy/mutex global variable.)