Upgrade heapless 0.7.15 -> 0.8.0

Summary: Sync with fw-rust.

Reviewed By: zertosh

Differential Revision: D51963086

fbshipit-source-id: 22021abffef9d8815f98b0c6fc3c9d4e3971331f

Author: rjbailey

experimental/reverie-sabre/Cargo.toml

@@ -10,7 +10,7 @@ license = "BSD-2-Clause"
 [dependencies]
 array-macro = "1.0.5"
 atomic = "0.5.1"
-heapless = "0.7.15"
+heapless = "0.8.0"
 lazy_static = "1.4"
 libc = "0.2.139"
 mimalloc = { version = "0.1", default-features = false }