feat(ci): Remove support for features after MSRV

[ratankaliani]

Motivation

Currently in kona, the CI runs with the latest stable toolchain.

This allows for features stabilized after the latest MSRV (e.g. rust-lang/rust#129195) to be used, which causes crates such as OP Succinct to fail to compile. This is because SP1's Rust toolchain (as well as other zkVM Rust toolchains) are not always up to date with the latest stable Rust toolchain, though they're above the MSRV in kona, which is currently set to 1.81.

Solution

• Update CI to use the MSRV toolchain (1.81).
• Add a rust-toolchain.toml file, which ensures during local development that the MSRV toolchain is used.
• Update functions to reflect the fact that const_mut_refs is not stabilized.
• Set clippy in justfile to use stable, instead of nightly. fmt should still use nightly for features enabled in rustfmt.toml.
• Remove #![allow(clippy::unnecessary_map_or)], which does not exist in the clippy toolchain for 1.81

Misc

• Expose start_server, which is nice for OP Succinct. Happy to separate this into a different PR.

Context

SP1's current supported Rust toolchain is 1.82, and the const_mut_refs feature was stabilized in 1.83.

[sp1]  error[E0658]: mutable references are not allowed in constant functions
[sp1]    --> /home/ubuntu/.cargo/git/checkouts/kona-eb08956031b8bea7/2e37ea5/crates/mpt/src/list_walker.rs:80:29
[sp1]     |
[sp1]  80 |     pub const fn take_inner(&mut self) -> Option<VecDeque<(Bytes, Bytes)>> {
[sp1]     |                             ^^^^^^^^^
[sp1]     |
[sp1]     = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
[sp1]     = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
[sp1]  
[sp1]  error[E0658]: mutable references are not allowed in constant functions
[sp1]    --> /home/ubuntu/.cargo/git/checkouts/kona-eb08956031b8bea7/2e37ea5/crates/mpt/src/list_walker.rs:81:9
[sp1]     |
[sp1]  81 |         self.inner.take()
[sp1]     |         ^^^^^^^^^^
[sp1]     |
[sp1]     = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
[sp1]     = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
[sp1]  
[sp1]  error: `core::option::Option::<T>::take` is not yet stable as a const fn
[sp1]    --> /home/ubuntu/.cargo/git/checkouts/kona-eb08956031b8bea7/2e37ea5/crates/mpt/src/list_walker.rs:81:9
[sp1]     |
[sp1]  81 |         self.inner.take()
[sp1]     |         ^^^^^^^^^^^^^^^^^
[sp1]     |
[sp1]     = help: add `#![feature(const_option)]` to the crate attributes to enable

[ratankaliani]

@clabby Let me know if you want me to consolidate the toolchain declaration as 1.81 in a cleaner way. This is the main issue blocking us from using the upstream, and would help us move off of our fork.

FWIW, we're generally within 2 versions of the latest stable Rust toolchain, and it's on our todo list to add 1.83 support. This PR is warranted IMO, because the MSRV declared inside of the Cargo.toml isn't being respected.

[clabby]

Ran CI over in #1021, looks good. Need to figure out what's going on with codecov tokenless upload. Thanks :)