Revamp handling of function types, conversion to function ptr types.

Implicit conversion to function types is now a more standard part of
the type conversion infrastructure, rather than special cases of things
like FunctionSymbolExpr immediately returning a pointer type, etc.

Improved AddressOfExpr::TypeCheck() to actually issue errors in cases
where it's illegal to take the address of an expression.

Added AddressOfExpr::GetConstant() implementation that handles taking
the address of functions.

Issue #223.

expr.h

@@ -584,6 +584,7 @@ public:
     Expr *TypeCheck();
     Expr *Optimize();
     int EstimateCost() const;
+    llvm::Constant *GetConstant(const Type *type) const;
 
     Expr *expr;
 };