Suggestion: `no_std` support (with `alloc`)

[gkgoat1]

I am writing a compiler for WASM and want to use e-graphs (particularly this crate) to optimize the output. However, the compiler is no_std, so this crate would need to be ported to be no_std compatible to work in it. Using alloc would be a possibility to start, as we don't use any OS functions in egg afaik.