Sycamore v0.8 only runs on Rust 2021.

Still have to do:
- `router/`
- `template/`
- `shell.rs`
- One tricky error in `error_pages.rs`

I removed the troublesome function in `error_pages.rs`, so that may bite
me soon.

Still have problems with `bind:value` (as we will in all the state
platform examples for now).

The remaining errors will be fixed after `make_rx` can work with
non-`Rc` `Signal`s.

Nothing works without hydration disabled though...

This *should* make the state platform work with lifetimes.

I have been shown a much better way of achieving the same outcome.

It will be easier to manually undo changes to make sure we preserve some
good things.

This reverts commit 15187b5.

This avoids a huge number of lifetime issues, and actually ends up being
more performant, without compromising on ergonomics.

This should make Perseus several orders of magnitude more ergonomic, in
line with Sycamore's new no-clones system!

This requires an irritating change to import practices unfortunately,
but the convenience and ergonomics are worth it.

This also involved some minor changes to the macros to fix some nested
state issues.

This improves ergonomics and makes the auth example compile.

* refactor: simplify provide_context_signal_replace

Also slightly improves performance in only making a single call to use_context

* fix: do not insert hydration keys in the head string

* chore: remove perseus/hydrate feature from Cargo.toml

* chore: merge imports for consistent code style

* fix: update sycamore to v0.8.0-beta.5 and remove workaround

Co-authored-by: arctic_hen7 <arctic_hen7@pm.me>

These were just for the demos that weren't ready at the time of the PR.

Hydration still doesn't work in the `auth` example.

This should fix the issues with the `body` element.

Also locked the old v0.3.4-5 docs to a specific commit hash, which keeps
the examples there safe to use.