Easy-to-use linear types
========================

Allow simple usage of linear types when using only local variables
and/or parameters and/or return values.

Have a "lock system" with automatic garbage collection (collected no later
than when the arena is destroyed) for more complex cases.


Assume that `open` create a linear/owned `File` type.

Trivial case:

    func stuff
    code
        own File f = open "file.txt" .write
        write f "hello"     # uses `f` but does not take ownership
        close f             # takes ownership of `f`
    end

Nested case:

    func get_file
    returns
        own File f
    code
        # Allow simple usage when returning
        return open "file.txt" .write
    end

Complex case:

    func maybe_open_file
    code
        if use_file
            # 
            begin_ownership thefile
            thefile = open "file.txt" .write
            suspend_ownership thefile
            # disallow changing the `file` variable when it is
            # suspended?
        end
    end

    func maybe_write_to_file
    code
        if use_file
            resume_ownership file
            write thefile "hello"
            suspend_ownership file
        end
    end

    func maybe_close_file
    code
        if use_file
            resume_ownership thefile
            close thefile
            end_ownership thefile
        end
    end