Commit Cargo.lock. (#705)

* Commit Cargo.lock.

The Cargo documentation recommends packages which produce end-user
programs, which wasmtime-cli is, to check in their Cargo.lock files:

https://doc.rust-lang.org/cargo/faq.html#why-do-binaries-have-cargolock-in-version-control-but-not-libraries

* Use Cargo's newer less-conflicting lockfile format

.gitignore

@@ -4,7 +4,6 @@
 *.swx
 tags
 target
-Cargo.lock
 .*.rustfmt
 cranelift.dbg*
 rusty-tags.*