refactor(core): `is_context_changed()` into two functions

Refactors `is_context_changed()` into two functions,
`get_context_change()` and `get_context_items_change()`, to eliminate
use of `goto` for cleanup, and more accurately reflect the purpose of
the function. Also renames some variables referencing the return value
from these functions.

These functions would probably benefit from unit tests proving them.