Replace individual bool bitfields in Object (m_is_extensible,
m_has_parameter_map, m_has_magical_length_property, etc.) with a
single u8 m_flags field and Flag:: constants.
This consolidates 8 scattered bitfields into one byte with explicit
bit positions, making them easy to access from generated assembly
code at a known offset. It also converts the virtual is_function()
and is_ecmascript_function_object() methods to flag-based checks,
avoiding virtual dispatch for these hot queries.
ProxyObject now explicitly clears the IsFunction flag in its
constructor when wrapping a non-callable target, instead of relying
on a virtual is_function() override.
This has quite a lot of fall out. But the majority of it is just type or
UDL substitution, where the changes just fall through to other function
calls.
By changing property key storage to UTF-16, the main affected areas are:
* NativeFunction names must now be UTF-16
* Bytecode identifiers must now be UTF-16
* Module/binding names must now be UTF-16
~2% of the Speedometer 2.1 profile was just repeatedly performing the
shape transitions to add these two properties. We can avoid all that
work by caching a premade shape.
