runtime: Add a basic monomorphic cache implementation

Tasks:

• [x] Allocate an inline cache inside ExecutableMap
• [x] Put receiver map-method object pairs inside the inline cache
• [ ] Query the inline cache during method lookup and use it first
• [ ] Invalidate the inline cache when the dependent method changes
• [ ] Make the inline cache weak, so that references can be dropped when they are no longer referenced by anything else

Optional tasks:

• [ ] Implement quickening (https://bernsteinbear.com/blog/inline-caching-quickening/)

Questions to answer:

• [ ] Who owns the inline cache?
• [ ] Should the inline cache be an ArrayObject or should it be given its own dedicated object type?
• [ ] Should global objects' inline caches be affected by actor-owned methods?
• [ ] Should actors' inline caches be affected by actor-owned methods?
• [ ] Implications of inline caching from a concurrency standpoint