Add clap-3.1.6 as a new benchmark.

This is a little faster to compile than the old `clap-rs` benchmark.
```
        clap-rs   clap-3.1.6
check   3.17s     2.78s
debug   4.44s     4.00s
opt     5.79s     5.16s
```
`clap-rs` will be removed soon.

Author: nnethercote

collector/benchmarks/README.md

@@ -17,6 +17,8 @@ They mostly consist of real-world crates.
 
 - **cargo-0.60.0**: The Rust package manager. A large program, and an important
   one in the Rust ecosystem.
+- **clap-3.1.6**: A command line argument parser. A crate used by many Rust
+  programs.
 - **clap-rs**: A command line argument parser. A crate used by many Rust
   programs.
 - **cranelift-codegen**: The largest crate from a code generator. Used by

collector/benchmarks/clap-3.1.6/.cargo_vcs_info.json

@@ -0,0 +1,6 @@
+{
+  "git": {
+    "sha1": "e702f42e5ec416b477dfc8f71d8caef3a3dff923"
+  },
+  "path_in_vcs": ""
+}

collector/benchmarks/clap-3.1.6/0-println.patch

@@ -0,0 +1,13 @@
+diff --git a/src/output/usage.rs b/src/output/usage.rs
+index 45cfa746..5e6431ce 100644
+--- a/src/output/usage.rs
++++ b/src/output/usage.rs
+@@ -15,6 +15,7 @@ pub(crate) struct Usage<'help, 'cmd> {
+ 
+ impl<'help, 'cmd> Usage<'help, 'cmd> {
+     pub(crate) fn new(cmd: &'cmd Command<'help>) -> Self {
++        println!("testing!");
+         Usage {
+             cmd,
+             required: None,
+