* Avoid pinning `storage_head` in `iseq_mark_and_move`

This refactor changes the behavior of `iseq_mark_and_move` to avoid
pinning the `storage_head`. Previously pinning was required because they
could be gc'd during `iseq_set_sequence` it would be possible to end
up with a half build array of instructions. However, in order to
implement a moving immix algorithm we can't pin these objects so this
rafactoring changes the code to mark and move. To accomplish this, it was
required to add `iseq_size`, `iseq_encoded`, and the `mark_bits` union
to the `iseq_compile_data` struct. In addition `iseq_compile_data` sets
a bool for whether there is a single or list of mark bits. While this
change is needed for moving immix, it should be better for Ruby's GC
as well.

* Don't allocate mark_offset_bits for one word

If only one word is needed, we don't need to allocate mark_offset_bits
and can instead directly write to it.

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Co-authored-by: Peter Zhu <peter@peterzhu.ca>