It's now possible to assign to a key of a dictionary and even to a nested
key path.
This patch changes the way matchers work a bit:
First, a function call stack frame now has a stack of matchers that are
manipulateable instead of a single matcher.
Second, the matcher is now in charge of setting the matched values to the
variables (previously the caller of the matcher needed to extract the
matched values and assign them itself). This change simplifies code
generation, especially for chained assignments and dictionary key paths.
This removes the last usage of APFL_ERR_NOT_IMPLEMENTED :)
