Support for Stable Toolchain

I suspect the answer is *yes*, but... is `nightly` in `rustup-toolchain.toml` required? If so, would some contribution be able (and welcome) to remove that requirement?

From my read of the code, it seems like this is required for dynamic expressions in macro bodies. Is that something that could theoretically be feature gated, or is that core to the functionality of this project?

Thanks for the great work here!

Start with rustup-toolchain.toml and the code handling dynamic expressions in macro bodies; determine which nightly-only capability requires the current toolchain. Check whether that capability can be feature-gated without affecting core functionality, then verify the project builds and its tests pass on stable Rust.

Written by the indexing model from the issue text.