Fuzz fixes

[graydon]

This contains the results of a few days of both reproducing and fixing fuzz bugs and also hacking on the fuzzer:

1. Fixes fuzz found internal error deleting from instance storage #1203 which was a somewhat subtle budget-error-escalation bug the fuzzer found.
2. Fixes a couple other instances of that pattern.
3. Fixes a fuzzer-found instance of a frame invariant corruption.
4. Moves the machinery the fuzzer needs from tests/utils (which was guarded by cfg(test) so inaccessible to testutils and guards it with cfg(feature="testutils")
5. Removes the duplicate code in the fuzzer lib and instead uses host testutils
6. Extends testutils to record the storage map and then switch to enforcing, rather than manually building the enforcing map
7. Removes the other non-expr fuzz case, it's redundant.
8. Adds some scaffolding for fuzz debugging.
9. Adds support for passing arguments to the fuzzer to enable access to storage and cross-contract calls.
10. Fix the thing where we rebuild every time due to build.rs not being conservative enough