adjust rust-version declaration

Cargo.toml

@@ -10,7 +10,7 @@ license = "BSD-3-Clause"
 keywords = ["prolog", "prolog-interpreter", "prolog-system"]
 categories = ["command-line-utilities"]
 build = "build/main.rs"
-rust-version = "1.57"
+rust-version = "1.61"
 
 [features]
 num = ["num-rug-adapter"]
@@ -79,4 +79,4 @@ modular-bitfield = { git = "https://github.com/mthom/modular-bitfield" }
 num-rug-adapter = { git = "https://github.com/mthom/num-rug-adapter" }
 
 [profile.release]
-debug = true
+debug = true