Replace mpmc_large feature using LenType

[bmgxyz]

• Remove mpmc_large feature
• Add new generic LenT to Sealed (supertrait of LenType) and propagate to mpmc::Queue, mpmc::QueueInner, and storage::SealedStorage; set LenT = u8 in for Queue and QueueInner to maintain backwards compatibility
• Add LenType::Atomic and LenType::Signed to support transition to LenT
• Add Atomic trait that shadows Atomic in the standard library, which is currently unstable
• Update and expand tests in mpmc and add a new cfail test

This PR introduces breaking changes.

Closes #561

LLM Usage Disclosure: I used an LLM to plan my changes, find solutions for certain compiler errors, and write test boilerplate. I wrote about half of the code by hand and reviewed all code myself.

[bmgxyz]

Is it necessary to update the benchmark? I would do it myself, but I don't know how.