Significantly reduce the tendrils of DeclSpecs/Declarator/Declaration code

The stuff in decl.h/decl.cpp is messy, largely due to its close mapping
to C-style variable declarations.  This checkin has updated code throughout
all of the declaration statement, variable, and function code that operates
on symbols and types directly.  Thus, Decl* related stuff is now localized
to decl.h/decl.cpp and the parser.

Issue #13.

ast.h

@@ -80,7 +80,8 @@ class AST {
 public:
     /** Add the AST for a function described by the given declaration
         information and source code. */
-    void AddFunction(DeclSpecs *ds, Declarator *decl, Stmt *code);
+    void AddFunction(Symbol *sym, const std::vector<Symbol *> &args, 
+                     Stmt *code);
 
     /** Generate LLVM IR for all of the functions into the current
         module. */