Builds targeting AVR fail with an error

[rihardsk]

If I add postcard to a project targeting AVR, I get an error

error: cannot find macro `llvm_asm` in this scope
   --> /home/rihards/.cargo/registry/src/github.com-1ecc6299db9ec823/critical-section-0.2.7/src/lib.rs:127:13
    |
127 |             llvm_asm!(
    |             ^^^^^^^^

The critical-section dependency is transitively brought in by heapless. There's already a corresponding issue in their repo – rust-embedded/heapless#312.

The solution seems to be to wait for heapless to make a new release and switch to using it.

[jamesmunns]

Cool! Feel free to @ me or open a PR when there is a new release, and I can bump the heapless dep.