Code quality improvements for v0.3.0+

[mxk]

General

• [x] Split README.md and crate documentation?
• [ ] General documentation cleanup.
• [ ] Simplify Error types and review the design.
• [ ] Review logging structure and maybe switch to span-based logging in places.
• [ ] Replace uses of std with core/alloc to start moving toward no_std support.
• [ ] Remove unnecessary derives.
• [ ] Restrict visibility for internal library components.

USB

• [x] Put usb backend behind a feature flag.
• [x] Do not expose rusb::Error.

HCI

• [x] Populate hci::Conn::local_addr when the connection is created. This requires hci::Host to track advertising state commands.
• [x] Block on hci::Error::CommandQuotaExceeded.
• [x] Store DisconnectionComplete reason.
• [ ] Log informational events (e.g. ReadRemoteVersionInformationComplete, LePathLossThreshold).

L2CAP

• [x] Handle L2CAP Signal channel events.
• [x] Get rid of l2cap::Alloc and cache transfers in RawChan.

SMP

• [ ] Send encryption request when the client first connects.
• [ ] Handle SMP rate limiting and repeated pairing attempts.
• [ ] Atomic file storage.
• [ ] Review smp::{Display, Confirm} API.

HID

• [x] Review HID module design and simplify for the new API.
• [x] Use physical DPI units for the mouse.