* Implement `black_box` as intrinsic
Responsibility of implementing the black box is now lies on backend
* Remove some TODOs
* Update to nightly-2021-09-17
* CI: don't fail on warnings
* Implement basic support for inline assembly
* Disable LTO
We don't support it yet at all
* Handle `inout(reg) var` correctly
Turns out that `+` readwrite output registers cannot be tied with
input variables.
* Add limited support for llvm_asm!
* Handle CHANNEL correctly
* Add support for arbitrary explicit registers
* Handle symbols properly
* Add rudimentary asm tests
* Exclude llvm_asm! tests from tests runs
* Insert `__builtin_unreachable()` after diverging asm blocks
