Arena allocation keywords
-------------------------
(subject to change. see also own.txt)

Inspired by this:
http://www.barnowl.org/research/rc/

LRL could have an "arenaown" keyword, which tells that a pointer is owned
by the arena in which the object is allocated. For such pointers there
can be some safety rules:

  - Ownership can't be transfered away to another pointer outside of the arena
    (but the pointer can used without taking ownership).
  - A such pointer can't take ownership of an object allocated outside of the
    arena (but may point to an unowned object, or something in the constant
    data section, or a function pointer*).
  - By default, all function parameters marked with "arenaown" or "arena" must
    belong to the same arena.
  - A parameter marked "arena" will not take ownership but is used to enforce
    that "arenaown" parameters are also allocated in the same arena. A function
    that allocates something in a given arena, must have at least one pointer
    marked with "arena" or "arenaown" and must pass it to the allocator
    function.
  - Future versions of LRL may add a special syntax to indicate which arena
    something is owned by.


* actually, even function pointers and pointers to the data section are risky
  since they may be part of a dynamic library, which can be unmapped from
  memory.


Code that doesn't use static variables, and at least one non-null pointer
parameter, can instead use a transparent arena allocation (by allocating
in the same arena as the parameter).