[libj6] Add thread wrapper class

This new class makes it easier for user programs to spawn threads. This
change also includes support for .hh files in modules, to differentiate
headers that are C++-only in system libraries.

scripts/bonnibel/source.py

@@ -49,7 +49,7 @@ class ParseSource(Source):
     @property
     def gather(self):
         _, suffix = splitext(self.output)
-        return suffix in (".h", ".inc")
+        return suffix in (".h", ".hh", ".inc")
 
     @property
     def next(self):
@@ -113,7 +113,7 @@ def make_source(root, path):
         return CompileSource(path, root)
     elif suffix in (".cog",):
         return ParseSource(path, root)
-    elif suffix in (".h", ".inc"):
+    elif suffix in (".h", ".hh", ".inc"):
         return HeaderSource(path, root)
     else:
         raise RuntimeError(f"{path} has no Source type")