Make `{core, io, common}` no_std and WASM compatible

gadget-{core, io, common} can now build in #![no_std] and wasm32-unknown-unknown environments.

Notable changes

Since this requires Error in core, I set the channel to nightly. This isn't long term, though. The feature should be out in a couple weeks.
- To go with that, the url and thiserror crates use no_std compatible forks for now. Unfortunately, it'll probably be awhile before those get merged.
gadget-common now builds with any combination of --no-default-features, and the std, substrate, and wasm features.
- This is pretty messy, though. common will need to be reorganized eventually or this compatibility will be a pain to maintain in the future.
gadget_core::{job, gadget} have been split into multiple modules to make it easier to gate their managers behind std.
Removed some usage of color_eyre.
- I haven't completely gotten rid of it yet, since it wasn't necessary to. We should be using thiserror instead, as eyre isn't meant for library errors.
- For now, I've just been returning String errors in its place. I left TODOs to make concrete error variants later.

Outside of that, this is mostly just changing some dependency features and feature-gating.

Note that the commits won't work individually. I split them up per-crate to make it easier to review.