build: shrink size of Rust artifacts

This does not do panic=abort because to be effective, you also need
to rebuild std, and _that_ means rebuilding images and also installing
from rustup (because the packages we are using do not include std
sources).

But this will still shrink the size a fair bit.

Cargo.toml

@@ -13,9 +13,10 @@ license = "Apache-2.0"
 debug = 2 # full debug info
 
 [profile.release]
-debug = 1
-lto = "thin"
+codegen-units = 1
+debug = "line-tables-only"
 incremental = false
+lto = "fat"
 
 [profile.tracer-release]
 debug = 1 # line tables only