Ability to limit memory usage

[Bastacyclop]

Hi!

Having a memory usage limit for the Runner would be really useful. There is an e-graph size limit, but the correlation with memory usage is unclear (for example memory is also allocated when applying rewrite rules inside an iteration).
I've implemented such a feature in my Scala port but it relies on Java runtime features, and I am not sure what is the best way to achieve something similar in Rust.

[cospectrum]

To calculate the size of EGraph we need to know the size that each node occupies. The node is a Language trait at the moment. Therefore, we will need to add an additional method/trait heap_size(&self) -> ByteSize for a node somewhere.