By specializing this template and using the special empty JS::Value as a
marker for the `void` state, we shrink this very common class from 16
bytes to 8 bytes.
This allows bytecode instruction handlers to return their result in a
single 64-bit register, allowing tighter code generation.
These were *extremely* hot in profiles (noticed when looking at
disassembly).
Now that we've made the special empty JS::Value much harder to create
accidentally, we can feel better about turning these into ASSERT and
catching them in debug builds.
The special empty value (that we use for array holes, Optional<Value>
when empty and a few other other placeholder/sentinel tasks) still
exists, but you now create one via JS::js_special_empty_value() and
check for it with Value::is_special_empty_value().
The main idea here is to make it very unlikely to accidentally create an
unexpected special empty value.
There are now no users of the MUST_OR_THROW_OOM macro. Let's rename this
macro to indicate it may be used to propagate any internal error (such
as the call stack limit error) in places that would otherwise crash due
to a MUST/VERIFY invocation.
Note there's no actual functional change here, as we weren't able to
ensure the internal error was an OOM error previously.
