Take the first option from a list of table keys and matcher functions.
This operation will commit to the first table key that matches. If the
associated matcher fails, only that error will be propagated and the
other alternatives will not be matched.
If no keys match, an error message is generated explaining which keys
would have been accepted.
This is provided as an alternative to chaining multiple
Toml.Schema.reqKey cases together with Alternative
which will fall-through as a result of any failure to the next case.