Code quality improvements for v0.3.0+

[mxk]

General

• 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

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

HCI

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

L2CAP

• Handle L2CAP Signal channel events.
• 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

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