Add ADCs to MUX in ATmega8

Fix timer patches for ATmega8

Add patches for ATmega8

Update CHANGELOG

Add support for ATtiny404

Small fixes

Prepare 0.4.0

Rename `udebug` feature to `ufmt`

Update CHANGELOG

Cargo.toml: Update edition to 2021

We have recently (#104) dropped support for compilers that don't support
edition 2021. Make this explicit by enforcing edition 2021 in
Cargo.toml.

Update CHANGELOG

Update the bare-metal crate to 1.0.0

The new crate version generates much cleaner code due to #[inline] annotations.
This is an API break, because the CriticalSection + Mutex mechanism changes
from exterior reference lifetime to interior reference lifetime management.

Update CHANGELOG

irq: Optimize interrupt save/restore

Avoid unnecessary mask and branch instructions.

The basic reasoning behind this is that all other flags in the SREG can
be clobbered without ill effects. The restore() function is an
optimization fence and the compiler is not allowed to make assumptions
about memory or SREG state after execution.

This avoids an `and` and a `breq` instruction or similar in every
critical section.

While at it, also introduce a better API for manual IRQ-flag management.

Remove all llvm_asm macros

ci: Upgrade to a new compiler version

Add basic documentation for beginners

Update CHANGELOG

Adds patch for the timers on attiny167

Add support for ATmega164pa

Adds the ATDF file provided from Microchip, and makes changes analog to
290613454fbdc5e4ac98e53deccaf74dafc88963.
This has been tested on real hardware.

Signed-off-by: Tronje Krabbe <tkrabbe@jusst.de>
Signed-off-by: Oskar Munz <omunz@jusst.de>