Take advantage of associated type defaults once they hit stable Rust

[Veetaha]

I've tried the nightly feature #![feature(associated_type_defaults)] to get rid of repetitive type {Member} = Unset<member::{member}>; entries in impl blocks for Set{Member} typestates. This yielded a considerable compilation perf. improvement from 16% to 58% depending on the number of members. This is because it removes the quadratic growth of the number of associated type entries that we have today in the impl blocks.

See more details about my measurements in the comment under the tracking issue for associated_type_defaults: rust-lang/rust#29661 (comment).

Here is a commit where I tested this feature in bon (it's quite dirty, but proves the concept): f884ca9

This issue is blocked by the stabilization of associated_type_defaults.

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