* utils: RefCountedInternPool/RefCountedMap
First, introduce RefCountedInternPool, a reference counted intern pool of
Slice<const T>. Just acquire() a slice that you want and you're guaranteed to
get exactly one canonical value-equal Slice<const T> back.
Additionally, introduce the concept of NullValue to RefCountedMap. A NullValue
defines what should be considered an uninitialized value; by default, it's the
default value of that type (0 for ints, nullptr for pointers, etc). This allows
us to lazily-initialize values in the map. A client can acquire() a bunch of
different resources which will be initialized only when get(factory) is called.
If a client attempts to get() a value without specifying a factory, and the
value is not initialized (i.e. equal to NullValue{}()), RefCountedMap will
panic.
* utils: add unit tests for ref-counted collections
* utils: remove C++20 features, fix memory issue
* utils: remove RefCounted from InternPool
