rust: Compile with ThinLTO

This should achieve similar performance gains to "fat" LTO (which we
were previously using) while taking substantially less time to run
(over 20s saved on a Ryzen 9 5950X).

Part of zcash/zcash#6065.

Cargo.toml

@@ -109,7 +109,7 @@ default-features = false
 features = ["ansi", "env-filter", "fmt", "time"]
 
 [profile.release]
-lto = true
+lto = 'thin'
 panic = 'abort'
 codegen-units = 1