Use sentinel function for active bindings

[hadley]

public = list(
  blah = active_binding(function(value) {
    
  })
)

And eventually deprecate the active_bindings argument

[idavydov]

Maybe active_binding could have a read_only flag?

public = list(
  blah = active_binding(function() {
    
  }, read_only=TRUE)
)

And then there could be nice error-handling by default (similar to the goal of #100 and #99).

x$blah <- 0
# Error: blah is read-only