The previous Map equality code was assuming that the set of keys would
be identical between the two Maps. This change adds insertion-order
tracking for each key with its corresponding key. If one of the two keys
is an asymmetric equality obj, the keys are eq()'d, and if it succeeds,
the corresponding values are compared. Otherwise, the "main" key is
looked up directly in the other object; this is to prevent
similar-looking obj keys with different obj identities from comparing
equal.
Fixes#1432.
This allows custom equality testers to affect asymmetric matches.
This avoid suprises when combining addCustomEqualityTester with
objectContaining or arrayContaining.
Closes#1138
