feat: Add typing for Sequence() operation

Also move the signature of the semantic sort TYPE into its own
  file to facilitate sharing between Statix and Stratego.
  (Currently it is shared via symbolic link, but that may cause
  problems down the line; if/when it does, will have to look at
  physically copying the file into src-gen via an "Additional
  build step" using either Stratego or Ant.

  Also documents using Statix types from Stratego.

tests/emit_thrice.fos

@@ -1,4 +1,4 @@
-  stream << 72 + 87
+  stream << 'Some numbers: '
   stream << 88
     + 96
   99 + 12 >>