Sigils for mutabilty and arena access
=====================================

Sigils for mutability and arena access:
    ! for writable/mutable access
    @ for arena access
    @~ for arena+own access (destructor)

Syntax 1:
    arena Thing obj = .new(other@)
    obj.work@(123, result!)
    obj.field! += 123

Syntax 2:
    arena Thing obj = .new(other@)
    obj@.work(123, result!)
    obj!.field += 123

    - Easiest to implement?
    - Makes it clearer that "obj" is being modified.
    - More consistent syntax

How about write-only?
Add a different sigil for this? Syntax?
    obj!!   obj!&   obj!$
    obj!~   obj!%
Or use obj! for write-only and obj!! for read-write?
    - but reads are necessary when accessing thing inside fields,
      even if nothing is "read" on the "high-level".
    - allow "write-only" to read 
    - "output" vs "write-only"

Alternative solution:
- Don't use sigils, and let "the IDE" color objects differently
  depending on how they are accessed (read-only, read-write or arena)
- Requires improving some existing IDE or writing a new one :(
- Bad for people who don't want to use an IDE.

Sigils for types
----------------
Should sigils be used for types also, instead of var/arena/own/etc?
- In that case, should "ref" still be used?

Syntax idea 1:

    ref Thing! t
    ref Thing@ t
    ref! Thing t
    t.modify!()
    t.dostuff@()
    f!()

Syntax idea 2:

    ref !Thing t
    ref @Thing t
    !ref Thing t
    t.!modify()
    t.@dostuff()
    !f()

Syntax idea 3:

    ref Thing= t
    ref Thing@ t
    ref= Thing t
    t.=modify()
    t.@dostuff()
    =f()

Syntax idea 4:

    ref Thing= t
    ref Thing@ t
    ref= Thing t
    t.modify=()
    t.dostuff@()
    f=()