Remove Cargo.lock as recommended by Rust docs.

The Rust language documentation recommends not including the `.lock`
file if the crate represents a library - it's there primarily to support
reproducible builds for binaries.

https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html

Author: NlightNFotis

src/libcprover-rust/.gitignore

@@ -1,2 +1,5 @@
 # Ignore build artefacts folder
 target/
+
+# Ignore Cargo.lock as recommended by Rust docs
+Cargo.lock

src/libcprover-rust/Cargo.lock

@@ -6,7 +6,7 @@ description = "Rust API for CBMC and assorted CProver tools"
 repository = "https://github.com/diffblue/cbmc"
 documentation = "https://diffblue.github.io/cbmc/"
 license = "BSD-4-Clause"
-exclude = ["module_dependencies.txt"]
+exclude = ["module_dependencies.txt", "Cargo.lock"]
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html