Update crates available in the playground

Due to a change in wasm-bindgen [1], the "coverage" feature is being
turned on, probably because we are never running the Cargo feature
resolver (`resolver = 2`). Unfortunately, doing so appears to be a big
change to the top-crates code so we will probably be doing manual
editing for a while.

[1]: rustwasm/wasm-bindgen#4304

Author: shepmaster