Lint building of *Lib.sml files

[dnezam]

Since *Lib.sml files seem to only get built when theories depend on them, it is possible for them to bitrot without being caught by the regression test suite. This issues tracks the creation of a tool/linter that can catch these untested files and recommends to either delete them, or create a theory that depends on them.

One potential way to get this to work is to use build-sequence and the ability of Holmake to create a dependency graph, to create a list of all *Lib.sml files that are currently tested. Then, one should be hopefully able to find all *Lib.sml files that are currently flying under the radar. The basic idea is similar to what is being done in #1115.