has_key, has_equal_key --> check_key + has_equal_value()

[filipsch]

Build check_key() so that it produces a state that has_equal_value() can use. Checked this with the CDs, and they like the idea.

Currently, has_key()andhas_equal_key()` are used by 5 exercises in total, so not urgent, and removing these functions shouldn't be a problem.