Add an "inverter" concept, so that functions can generate lists of possible arguments

[julianhyde]

Consider the query

from s where String.isPrefix s "abcd";

We know how to invert the function and find the list of values of s such that String.isPrefix s "abcd" will return true. In fact it's a property of the String.isPrefix function.

So, we allow functions to declare inverters. In this case, the String.isPrefix function accepts the argument pattern "f a0 a1", and if argument 0 is the goal - the variable whose possible values we wish to enumerate - then we can generate the expression prefixesOf a1.

At first only built-in functions will be able to declare inverters. But later we will allow user-defined functions to define them. We may also convert existing logic - e.g. that a0 elem a1 can be inverted and a1 is the list of possible values of a0 - into inverters.

[julianhyde]

Work in progress: https://github.com/julianhyde/morel/tree/217-inverter