Specify exact versions of all dependencies.

This commit does the following.

- Adds a `Cargo.lock` file to those benchmarks that lack one. This
  ensures that what we are measuring doesn't change. These files were
  generated by Cargo 0.8.0, which is the stable version on my Linux box.

- Removes the `rm Cargo.lock` command from all the `clean` make targets.

Author: nnethercote

futures-rs-test-all/Cargo.lock

@@ -5,5 +5,4 @@ all:
 touch:
 	rm -f target/debug/all-*
 clean:
-	rm target -rf
-	rm Cargo.lock
+	cargo clean

futures-rs-test-all/makefile

@@ -6,4 +6,3 @@ touch:
 	find . -name '*.rs' | xargs touch
 clean:
 	cargo clean
-	rm Cargo.lock

helloworld/Cargo.lock

@@ -6,4 +6,3 @@ touch:
 	find . -name '*.rs' | xargs touch
 clean:
 	cargo clean
-	rm Cargo.lock