Update heapless requirement from 0.4.1 to 0.5.1

Updates the requirements on [heapless](https://github.com/japaric/heapless) to permit the latest version.
- [Release notes](https://github.com/japaric/heapless/releases)
- [Changelog](https://github.com/japaric/heapless/blob/master/CHANGELOG.md)
- [Commits](rust-embedded/heapless@v0.4.1...v0.5.1)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

Cargo.toml

@@ -39,7 +39,7 @@ panic-itm = "0.4.0"
 cortex-m-rtfm = "0.4.0"
 cortex-m-semihosting = "0.3.2"
 enc28j60 = "0.2.0"
-heapless = "0.4.1"
+heapless = "0.5.1"
 libm = "0.1.2"
 mfrc522 = "0.2.0"
 as-slice = "0.2.0"