Use sidemap for GC object metadata

Zebu intends to side maps to seperate objects from their metadata.

However, currently as a compromise, I am using a 64-bit object header along with GCType for the metadata. We will go back to using side maps.

This issue describes the design of sidemap scheme.

• we will assume a minimal object size MIN_SIZE, and a minimal alignment MIN_ALIGN. The larger the minimal size/align is, the less memory is required for metadata (see below). However, it wastes memory in the heap. MIN_SIZE of 16/24/32bits bytes, MIN_ALIGN of 128bits are reasonable.
• object metadata includes:
  • 1bit/MIN_ALIGN: object start (and end - so we can decide size) (size is required for copying/dumping object)
  • 1bit/64bits: reference locations
  • 8bits/MIN_SIZE: gc state (mark bit, reference count, etc)
• small objects have less space to encode metadata, but large objects have plenty. We can use different schemes to encode small/large objects.
• side maps should be stored in the metadata part of a page/memory chunk.

More concrete design will be updated here once we discuss more.