Set z3 memory limit

[Daejun Park]

For some nonlinear queries, the peak memory usage of z3 may be quite high. If the system memory is not enough (e.g., the Github-hosted ubuntu and windows runners), such z3 process may be killed due to OOM, crashing halmos.

To prevent this, introduce an additional option, say --solver-max-memory, that sets the max memory of z3:

solver.set(max_memory=<size-in-megabytes>)

Also, set the default max memory of z3, based on the system memory size, e.g., using psutil.

[Daejun Park]

Related to #125

[Antonio Viggiano]

I think max_memory could be automatically set by default to e.g. 80%-90% of the system capacity. Then advanced users can override this value with --solver-max-memory