Semantics: include takes one or more filenames expressed as string literals,
applies an implementation-specific algorithm to find corresponding files,
reads the contents of the files in the specified order as if by repeated applications
and effectively replaces the include expression
with a begin expression containing what was read from the files.
By default, Husk will search for files from the current directory. If a file is not found, Husk will attempt to load it from the Husk system directory.