Function libunseemly::language_from_file

pub fn language_from_file(path: &Path) -> Language

Run the file (which hopefully evaluates to capture_language), and get the language it defines. Returns the parse context, the type environment, the phaseless version of the type environment, and the value environment. This doesn’t take a language 4-tuple – it assumes that the language is in Unseemly (but of course it may do include /[some_language.unseemly]/ itself). TODO: we only need the phaselessness for macros, and maybe we can get rid of it there?