Add support for asm!()

[Patryk27]

Closes #254.
Closes #229.

Abstract & rest the same as for Rahix/avr-device#97 🙂

As for the LLVM IR, there's a small change in delay():

older rustc:
  %7 = tail call addrspace(0) i16 asm sideeffect "1: sbiw $0,1\0A\09brne 1b", "=w,0"(i16 %6) #10, !srcloc !6

newer rustc:
  tail call addrspace(0) void asm sideeffect alignstack "1:\0Asbiw ${0}, 1\0Abrne 1b", "w,~{sreg},~{memory}"(i16 %5) #14, !srcloc !7

... but I think it looks alright (& the code works fine on an Atmega328p).

[Rahix]

Here as well: thanks a lot for this change!

[piegamesde]

Thanks for the work. I still have the problem that embedded-hal v0.2.7 does not support the new asm macro though, and avr-hal-generic seems to be depending on it. I tried one of the 1.0.0-alpha.X versions of embedded-hal and they seem to work, however I can't locally override the dependency because of semver.

[Rahix]

Hm, that's odd, where does embedded-hal need inline assembly?

[piegamesde]

My bad, I read the compiler error wrong. It is avr-device which needs to be bumped

[Rahix]

Ah, yeah, that is true indeed. As soon as I have released a new version of avr-device, I will bump the dependencies here.