support termination of errors by __close()

[belm0]

If a __close() metamethod was called with an error in flight, the error can be terminated by returning true.

See use cases.

This is a work in progress, and the implementation may not be robust.

example (not yet working):

do
  local x <close> = setmetatable({}, {
    __close = function(self, err)
      print('__close()')
      return true
    end
  })

  error('oops')
  print('not reached')
end

print('goodbye')

__close()
goodbye

TODO:

• fix stack unwind not stopping at the to-be-closed level
• documentation

[well-in-that-case]

Maybe I've misunderstood the rationale, but I feel like this is a patch skirting around try .. catch syntax. A TBC variable being able to enforce such authority over an entire block of code is uncanny.