Lazily coercing might have made sense in the past, but since hashing
and comparing requires the `PropertyKey` to be coerced, and since a
`PropertyKey` will be used to index into a hashmap 99% of the time,
which will hash the `PropertyKey` and use it in comparisons, the
extra complexity and branching produced by lazily coercing has
become more trouble than it is worth.
Remove the lazy coercions, which then also neatly allows us to
switch to a `Variant`-based implementation.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
