Implementing += operator for record types.

This is per #375.

Record types can now get additional fields later via '+='. The added
fields must however either be &optional or have a &default value.

Example:

    type Foo: record {
        a: count;
        b: count &optional;
    };

    redef record Foo += {
        c: count &default=42;
        d: count &optional;
    };

    global f: Foo = [$a=21];

    print f;

Output:

    [a=21, b=<uninitialized>, c=42, d=<uninitialized>]

src/Type.h

@@ -409,6 +409,10 @@ public:
 
 	int NumFields() const			{ return num_fields; }
 
+	// Returns 0 if all is ok, otherwise a pointer to an error message. Takes
+	// ownership of list.
+	const char* AddFields(type_decl_list* types);
+
 	void Describe(ODesc* d) const;
 	void DescribeFields(ODesc* d) const;