Consider isolate group-wide weak cache for RegExp

[mkustermann]

When creating RegExp objects, the factory constructor looks up in a cache and may return an existing regexp if available, otherwise creates a new _RegExp object, puts it into the cache and returns it.

The cache itself is a custom hashmap implementation with hard-coded maximum size of 256. So that means we hold on to regexp objects even if we don't need them anymore, but we also may fail to re-use existing regexp objects if the cache is exhausted. Furthermore the cache isn't shared across isolates, and may therefore lead to multiple regexps for the same pattern.

We could piggy-back on the recently introduced weak arrays: Insert RegExp objects into a weak array based hash set. New regexp constructor invocations can look up in the weak set for existence of an existing entry and re-use. It could be shared across isolates since regexp's themselves can be shared. (somewhat analogous to weak string canonicalization table).

(It would also be nice to remove yet-another custom set/map implementation)

/cc @aam @rmacnak-google