[std] Standardize on a common set of post-build steps

[kylewlacy]

For packages in brioche-packages, most builds need to do some "post-build" processing: things like setting env vars when used as a dependency using std.setEnv(), setting an entrypoint with std.withRunnableLink(), etc. The exact set of things that need to be done are different for every package of course, but we should provide clear instructions on what post-build steps packages should use.

This is a very high-level issue that can take many forms:

• Writing docs to cover which utilities packages should use and when
• Adding utilities to wrap the "common" options under one name (e.g. a function like std.package() that has a bunch of options to apply the most common utilities)
• Figuring out new Brioche features that could automatically handle some of the utilities automatically (e.g. a "post-build" event handler, which could wrap the output of a build with another recipe to automatically set env vars like $LIBRARY_PATH, $PKG_CONFIG_PATH, etc)

[kylewlacy]

This is probably related to #135. On the same note, we should also look at fixing libtool files as well in this step: see #151 for an example where our existing libtool files caused trouble