bench: split to separate package

Unfortunately cargo doesn't allow optional dev dependencies (see
rust-lang/cargo#1596) so this is the workaround.

Previously, running `cargo test` triggered the build of all dev
dependencies, including criterion, which in turn pulls in a lot of
stuff. Running tests should be faster, hence this change.

Cargo.toml

@@ -1,5 +1,5 @@
 [workspace]
-members = ["aoc*"]
+members = ["aoc*", "aoc*/aoc*_bench"]
 
 default-members = ["aoc2020"]