🎬 Encode the start function in ModuleData rather than the linker

[acfoltzer]

This long-overdue refactoring moves the metadata required to get a module's Wasm start function to the lucet-module types, rather than encoding it directly as a pointer-to-a-pointer in the shared object's symbols.

Of particular benefit is the fact that once a DlModule is created, we no longer use the underlying libloading::Library. Before the version bump in this patch, that library had a mutex around calls to dlerror to try preventing races between all threads in the process, which could lead to contention when threads attempted to concurrently use Lucet instances.

[acfoltzer]

We could definitely just use libc to call libdl functions directly, but I would probably want to write wrappers around them that would turn out very very similar to what libloading provides. But eventually just getting rid of it entirely would be :100: