[Unity] Various improvements (#67)

Several improvements were made to the Mono and IL2CPP code that should allow for easier mantainance and less spaghetti code:
- Mono was already using the `mono_assembly_foreach` symbol to find the game assemblies. This code is now part of the crate itself so the relative call has been changed
- PE module sizes are now queried using `pe::read_size_of_image` in order to avoid issues with Proton/Wine under linux
- The function to automatically detect the IL2CPP stuct version to use was broken. It has been fixed now, although there is no guarantee this will work fine on every game, so help by experienced Unity hackers might be appreciated anyway
- A `CSTR` const is now used instead of manually defining a capacity of 128 every time an `ArrayCString` is read from memory. This way, in case of need, the capacity of the `ArrayCString`s can be changed as needed
- A (private) `Assembly` struct has been added, as a `MonoImage` is technically loaded from the assembly. There is no need to make this struct public, but it allows to streamline some code. Also, if more functions are needed that rely on an `Assembly` instead of a `Image`, this should allow for easier future implementations
- A `Field` struct has been added in order to use some functions specific for `MonoField` (`get_name()` and `get_offset()`). `Class::get_field` has been renamed to `Class:get_field_offset` for better clarity. As for `Assembly`, this struct is not exposed publicly
- Code has been slightly modified and optimized, for example by using iterators instead of standard loops
- Some functions have now a simpler syntax
- Empty iterators now properly return as empty iterators, instead of returning Error
- A `UnityPointer` struct has been added to allow for automatic pointer path resolution. It internally uses `DeepPointer`, wrapped into a `OnceCell` to allow for lazy evaluation of the pointer path with interior mutability when trying to `deref()` it. Example:

```rust
    let is_loading = UnityPointer::<5>::new("SceneManager", 1, &["s_sInstance", "m_bProcessing"]);

    ...

    watcher.is_loading.update(is_loading.deref::<bool>(&process, &module, &image).ok());

```