Use `Arc<str>` instead of `String` for `Symbol`

[arthurpaulino]

[Post #364]

This will enhance the speed and memory consumption on every routine that involves cloning symbols. For reference, this idea is already implemented in src/lem/symbol.rs, which should be removed in favor of src/symbol.rs after this change.

[huitseeker]

@arthurpaulino can we close this?

[arthurpaulino]

This was originally @gabriel-barrett's idea and is not implemented yet. So I defer the answer to him