Use new identifier forest API

[BruJu]

• Use new identifier forest API from Identifier Forest Rewrite wasmify-sophia#10
• Intersection / Union / Difference / Contains implementations are moved to the identifier-forest crate
• quad_forest and lib.rs files merged for simplicity
  • lib.rs was basically empty
  • the wasm-tree-backend crate isn't supposed to do a lot of thing, except providing a thin wrapper around identifier-forest with some WebAssembly / WasmTreeFrontEnd specific small extra features

When/if this new identifier forest API will be/is stabilized and merged, this PR will be ready to be merged (all the front end unit tests when using the new front-end pass).