Include Cargo.lock file in git

This should make builds (especially for MSRV) much more stable, at the cost of no longer testing schemars compatibility with new dependency versions.

.gitignore

@@ -2,9 +2,5 @@
 # will have compiled files and executables
 /target/
 
-# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
-# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
-Cargo.lock
-
 # These are backup files generated by rustfmt
 **/*.rs.bk