Upgrade to clap 3.

This requires moving away from the deprecated `clap_app!` macro. Clap's
derive API (formerly `StructOpt`) is nicer anyway, because it's not
stringly-typed.

There is quite a bit of furniture moving in `execute.rs` to better
separate the perf tools used for `bench_*` subcommands from those used
for the profile subcommands.

Author: nnethercote

Cargo.lock

@@ -1,11 +1,12 @@
 [package]
-authors = ["Mark Simulacrum <mark.simulacrum@gmail.com>"]
+authors = ["The Rust Compiler Team"]
 name = "collector"
 version = "0.1.0"
-edition = '2018'
+edition = "2018"
+description = "Collects Rust performance data"
 
 [dependencies]
-clap = "2.25"
+clap = { version = "3.0.6", features = ["derive"] }
 env_logger = "0.8"
 anyhow = "1"
 thiserror = "1"