This patch makes it possible for JS::Object::internal_set() to populate
a CacheablePropertyMetadata, and uses this to implement a basic
monomorphic cache for the most common form of property write access.
This function must return true if the object may intercept and customize
access to indexed properties (properties where the property name is a
non-negative integer.)
This will be used to implement fast path optimizations for array-like
accesses in subsequent commits.
This function now takes an optional out parameter for callers who would
like to what kind of property we ended up getting.
This will be used to implement inline caching for property lookups.
Also, to prepare for adding more forms of caching, the out parameter
is a struct CacheablePropertyMetadata rather than just an offset. :^)
