feat: arbitrary solutions for thm/ax challenges (take 2)

[JasonGross]

This PR allows challenges that demand the construction of terms of
non-Prop types.

We remove the error "Challenge and solution constant kind don't match"
in favor of permitting any kind of solution to a challenge that is a
theorem or axiom.

Note that permitted axioms must still be declared as axioms or theorems
in the solution; it would invite inconsistency if we allowed
instantiating informative axioms with definitions.

The optional field "allow_partial" indicates whether or not challenges
may be solved with partial def solutions. We uniformly forbid unsafe
solutions. We rely on the Lean kernel to mark as unsafe (resp. partial)
any definitions that make use of unsafe (resp. partial) dependencies.

This is a more invasive and global change than #7, but I believe it is more principled and better designed. (And when you exclude the 21 loc in Comparator/Utils.lean which is present only for having nicer error messages, it is actually a net reduction of -10 loc.)

Axioms is now responsible for ensuring that solutions to challenges are present and have valid kinds (no forbidden axioms recursively; only partial if permitted; not unsafe)

Compare is is responsible for ensuring that:

• targets are present and are axioms or theorems in the challenge file
• targets have the same type in the challenge and solution
• any dependency of a solution, which is not an explicit target, and which occurs in both the challenge and the solution, has the same contents in both

Fixes #3 (mostly)
Fixes #4
Closes #5
Closes #6
Closes #7