* Improve the `wasmtime` crate's README
This commit is me finally getting back to #2688 and improving the README
of the `wasmtime` crate. Currently we have a [pretty drab README][drab]
that doesn't really convey what we want about Wasmtime.
While I was doing this I opted to update the feature list of Wasmtime as
well in the main README (which is mirrored into the crate readme),
namely adding a bullet point for "secure" which I felt was missing
relative to how we think about Wasmtime.
Naturally there's a lot of ways to paint this shed, so feedback is of
course welcome on this! (I'm not the best writer myself)
[drab]: https://crates.io/crates/wasmtime/0.37.0
* Expand the "Fast" bullet a bit more
* Reference the book from the wasmtime crate
* Update more security docs
Also merge the sandboxing security page with the main security page to
avoid the empty security page.
a75f383f96
