Lwtreslib: Split seq into tiered variants, add some missing functions, require explicit equality/comparison

Context

Part of !1872 (closed)
On top of !2026 (merged)

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Provide automatic testing (see the testing guide).
• N/A For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• opam branch: raphael-p@add-array-and-seq-variants-to-lwtreslib-opam