Enter/exit hooks

[thomaswilburn]

Was just thinking about this today re: a conversation about making AML safer in a mixed-skill newsroom environment, and these would be easy hooks to add--one with the keypath when the assembler adds an object or array to the tree, and another when it pops that branch off the stack. Especially in the latter case, when we exit the object, that's an ideal time to do validation of the item as a whole (checking for missing keys, validating user input, adding computed values).