LibJS: Encode Cell Values as conventional bottom-tagged pointers

[yyny]

Cell values were previously encoded as NaN-boxed pointers,
with the Cell type encoded in the top bits of the NaN-payload.

However, since Cell values are so common, this meant that we had to
do a lot of bit masking and bit shifting to retrieve the pointer
everytime we wanted to do a dereference.

Additionally, on all systems we support, (user-space) pointers must
always start with leading 0 bits.

The new encoding therefore takes advantage of this by storing pointers
without any NaN-boxing, and NaN-boxing the subnormal numbers
instead, which also all start with leading 0 bits.

Since subnormals are rarely seen in practice, it makes sense to do
the masking and shifting on these values instead, and encode the
more common Cell pointers in their usual encodings, thereby
increasing performance on average.

We can then further take advantage of the fact that Cell pointers are
8-byte aligned, and use the unused bottom bits to store the cell type.

This is called "bottom-tagging", which is better for modern CPUs as it
still allows them to prefetch the right cache line, since cache lines
are also >8-byte (usually 64-byte) aligned, and therefore have no reason
to inspect the bottom bits of a pointer in this case.

Bottom-tagging is also a more conventional approach, and as such
alternative garbage collector implementations are much more likely to
support it as compared to NaN-boxing.

Furthermore, since CellTag::Object == 0, Value::as_object() can
be optimized to a no-op in most cases, which makes the compiler and
the CPU very happy :^)

Note that since encoding non-userspace pointers is no longer supported,
LibJS will no longer work correctly in kernel-space and on some
embedded devices.

The capability to encode kernel-space pointers could be added back in
the future by adding a special case for them such that they will still
be NaN-boxed as before, at a performance cost of course.

[github-actions]

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