`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.

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