- Renaming sp_rcontext::sp to sp_rcontext:m_sp for consistency
  with other sp_rcontext_members, and for consistency with the
  same purpose member Item_sp_variable::m_sp.

- Passing a "const sp_head*" pointer to sp_rcontext::sp_rcontext()
  and to sp_rcontext::create().
  Initializing sp_rcontext::m_sp right in the constructor
  instead of having a separate initialization after "new sp_rcontext"
  or sp_rcontext::create().

- Adding the "const" qualifier to sp_rcontext::m_sp and Item_sp_variable::m_sp