Sanity test all solvers and fix `topdown_term`

[sim642]

We've kept various solvers around and compiling, but they're untested. For example, it turns out that topdown_term is completely broken and only computes bottoms.

Changes

1. Adds cram test for 00-sanity/01-assert to check minimal functioning of all solvers.
2. Fix start variables in topdown_term to restore basic functionality.
3. Fix invalid widen call in topdown_term for side effects.
4. Fix option name in topdown_space_cache_term.