tweak bitwise-operation optimization

Recognize arguments that are variables bound to fixnums.
meta/new-web: gzip ".css" and ".js" on upload to S3

We assume that any web browser where ".css" or ".js" useful
can also handle gzipped content. Compress files that are
at least 1k.

Also, use the new "shallow" mode of s3-sync to speed updates
to download.racket-lang.org.