Before this patch, every shape would permanently remember every other
shape it had ever transitioned to. This could lead to pathological
accumulation of unused shape objects in some cases.
Fix this by using WeakPtr instead of a strongly visited Shape* in the
the forward transition chain map. This means that we will now miss out
on some shape sharing opportunities, but since this is not required
for correctness it doesn't matter.
Note that the backward transition chain is still strongly cached,
as it's necessary for the reification of property tables.
An interesting future optimization could be to allow property tables
to get garbage collected (by detaching them from the shape object)
and then reconstituted from the backwards transition chain (if needed.)
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
