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.cpp

@@ -36,9 +36,7 @@
 */
 
 #include "ast.h"
-#include "decl.h"
 #include "func.h"
-#include "type.h"
 #include "sym.h"
 
 ///////////////////////////////////////////////////////////////////////////
@@ -52,8 +50,8 @@ ASTNode::~ASTNode() {
 // AST
 
 void
-AST::AddFunction(DeclSpecs *ds, Declarator *decl, Stmt *code) {
-    functions.push_back(new Function(ds, decl, code));
+AST::AddFunction(Symbol *sym, const std::vector<Symbol *> &args, Stmt *code) {
+    functions.push_back(new Function(sym, args, code));
 }