feat: add benchmarks for the IndexedCrate::new method (#403) (#408)

Currently uses `criterion` for benchmarks, but another option would be
`iai-callgrind` which provides "oneshot" benchmarks (which are way
faster and more reliable in busy systems like CI), but AFAIK
`iai-callgrind` only supports Linux.

Also adds a script to download aws-sdk-ec2 and build the rustdoc JSON.
It should work on MacOS, but I don't have a way to test that. It also
relies on `rustup` (because we use `cargo +nightly`).

Current numbers on my system:

```
IndexedCrate/new(aws-sdk-ec2)
                        time:   [1.5561 s 1.5609 s 1.5656 s]
```

Co-authored-by: Jalil David Salamé Messina <60845989+jalil-salame@users.noreply.github.com>

.gitignore

@@ -16,3 +16,8 @@ test_crates/**/Cargo.lock
 
 # ide
 .idea
+
+# Benchmarks and profiling related data
+flamegraph.svg
+perf.data
+perf.data.old