Explicit representation of ASTs for all the functions in a compile unit.

Added AST and Function classes.
Now, we parse the whole file and build up the AST for all of the
  functions in the Module before we emit IR for the functions (vs. before,
  when we generated IR along the way as we parsed the source file.)

module.h

@@ -40,10 +40,11 @@
 #define ISPC_MODULE_H 1
 
 #include "ispc.h"
+#include "ast.h"
 
 namespace llvm
 {
-class raw_string_ostream;
+    class raw_string_ostream;
 }
 
 class Module {
@@ -120,10 +121,9 @@ public:
     /** The diBuilder manages generating debugging information */
     llvm::DIBuilder *diBuilder;
 
-    GatherBuffer *gatherBuffer;
-
 private:
     const char *filename;
+    AST *ast;
 
     /** Write the corresponding output type to the given file.  Returns
         true on success, false if there has been an error.  The given