generation of debug info consumes a massive amount of memory and quite a bit of it seems to get lodged in the global llvmcontext meaning it becomes a leak in the current design of wavm. Removing this as we don't need it

After creating a ModuleInstance from a Module, we really don't need to hold on to the Module any longer. It's a memory sink. Refactor the one instance we needed the Module and free it after that

this lookup should be well protected; but no complaints to protect it further it's not a hot path or anything