runtime: Add a basic monomorphic cache implementation

[sin-ack]

Tasks:

• Allocate an inline cache inside ExecutableMap
• 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