LRAT signature (#2731)

LRAT signature (#2731)

* LRAT signature

Added an LRAT signature. It is almost entirely side-conditions, but it
works.

There is also a collection of tests for it. You can run them by invoking

```
lfscc smt.plf sat.plf lrat.plf lrat_test.plf
```

* Update proofs/signatures/lrat.plf per Yoni's suggestion.

Co-Authored-By: alex-ozdemir <aozdemir@hmc.edu>
* Responding to Yoni's comments.

* Removed unused varaibles

Some tests declared `var`s which were unused.
Now they don't.