build(deps): bump heapless from 0.7.4 to 0.7.5 (#705)

Bumps [heapless](https://github.com/japaric/heapless) from 0.7.4 to 0.7.5.
- [Release notes](https://github.com/japaric/heapless/releases)
- [Changelog](https://github.com/japaric/heapless/blob/master/CHANGELOG.md)
- [Commits](rust-embedded/heapless@v0.7.4...v0.7.5)

---
updated-dependencies:
- dependency-name: heapless
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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