`rp` and other commands were broken for me because they always showed
the object as a T_NONE.

The reason was that instead of returning the type `struct RBasic`,
FindFirstType("struct RBasic") was returning
`yjit::cruby::autogened::RBasic`.

Explicitly asking for the top-level RBasic by prefixing it with `::` is
enough to fix those commands.

In ruby/ruby#13008 `RVALUE` was removed without replacement. This means
the lldb scripts that relied on `RVALUE` stopped working.

I updated the ones that were using it just for the bytesize to use
`slot_size` and then round to the nearest power of 40. We can't use
`slot_size` directly because in debug mode it's `48` but `RVALUE` is
`40` bytes.

For the `as_type` method, I updated it to check the type. It's only used
for `bignum` and `array` so that's a simple change.

Lastly, for the `dump_page` method I replaced it with `struct free_slot`
since that's looking at the freelist.

`struct RVALUE` has been removed from all the scripts and I verified
that `rp` is fixed. I'm not confident the `dump_page` method is fixed,
the freelist looks off, but for now this gets us closer.

Now that we've inlined the eden_heap into the size_pool, we should
rename the size_pool to heap. So that Ruby contains multiple heaps, with
different sized objects.

The term heap as a collection of memory pages is more in memory
management nomenclature, whereas size_pool was a name chosen out of
necessity during the development of the Variable Width Allocation
features of Ruby.

The concept of size pools was introduced in order to facilitate
different sized objects (other than the default 40 bytes). They wrapped
the eden heap and the tomb heap, and some related state, and provided a
reasonably simple way of duplicating all related concerns, to provide
multiple pools that all shared the same structure but held different
objects.

Since then various changes have happend in Ruby's memory layout:

* The concept of tomb heaps has been replaced by a global free pages list,
  with each page having it's slot size reconfigured at the point when it
  is resurrected
* the eden heap has been inlined into the size pool itself, so that now
  the size pool directly controls the free_pages list, the sweeping
  page, the compaction cursor and the other state that was previously
  being managed by the eden heap.

Now that there is no need for a heap wrapper, we should refer to the
collection of pages containing Ruby objects as a heap again rather than
a size pool

For now, the old function still exists as `old_rp`, in order to debug
issues with this command.

This reverts commit 9a6803c90b817f70389cae10d60b50ad752da48f.

This reverts commit 68bc9e2e97d12f80df0d113e284864e225f771c2.