[meta] Refactoring related to Mmapper and page sizes

[wks]

We recently discussed some changes related to page sizes and mmapper. This issue keeps a check list for those issues and their dependencies.

• Refactor Mmapper to combine ByteMapMmapper and TwoLevelMmapper.
  • Refactor both Mmapper impls into ChunkStateMmapper #1369
  • This is a software-enginnering change and does not change the semantics of Mmapper.
  • This tidies up the code and should make it easier to make other changes.
• Remove the Protected state and dead methods related to it.
  • The currently unused Mmapper::protect #1348
  • It is better to tidy up Mmapper before doing this because this changes the semantics of Mmapper.
• Reduce the amount of code dependent on the page size.
  • Particularly, refactor methods of Mmapper so that they take the number of bytes as arguments. Concrete Mmapper implementations may work on a coarser granularity anyway.
• Introduce an os module to hold OS-dependent parts of mmtk-core.
  • This should include the page size which varies between OSes and architectures.