Consider limiting use of nightly features

[jstarks]

The code uses a bunch of nightly Rust features, most of which surely aren't strictly necessary (e.g., let chains support). This creates an ongoing maintenance burden to keep it building as upstream Rust evolves. E.g., this project doesn't build with Rust 1.86 seemingly just because some nightly-only MaybeUninit method got renamed in Rust 1.87.

I'd suggest limiting your nightly feature use only to what's necessary to achieve specific goals around no-std support or binary size, rather than just to be able to use cool new language/std library features a little early.

[lhecker]

The code uses a bunch of nightly Rust features, most of which surely aren't strictly necessary (e.g., let chains support).

Ironically, this is the only thing out of all the nightly features I would not want to miss out on. But it's been stabilized now so that problem will surely solve itself soon without me being proactive about it.

If anyone wants to submit shims for the other nightly features, please feel free to submit a PR (it would go into src/helpers.rs). The only thing that requires more work than the others is linked_list_cursors (needs a custom linked list class presumably?) and allocator_api (requires a custom Vec implementation in src/arena which would be fitting considering there's already a custom String class as well).