Created skeleton of a new, much simpler string type.

src/upb_string.h

@@ -0,0 +1,37 @@
+/*
+ * upb - a minimalist implementation of protocol buffers.
+ *
+ * Copyright (c) 2010 Joshua Haberman.  See LICENSE for details.
+ *
+ * This file defines a simple string type, which has several important features:
+ *
+ * - strings are reference-counted.
+ * - strings are logically immutable.
+ * - ...however, if a string has no other referents, it can be "recycled"
+ *   into a new string without having to free/malloc.
+ * - strings can be substrings of other strings (owning a ref on the source string).
+ * - strings can refer to un-owned memory; attempting to acquire a reference will
+ *   copy the data at that time.
+ */
+
+typedef struct _upb_string {
+  char *ptr;
+  uint32_t byte_len;
+  uint32_t byte_size;
+  upb_atomic_refcount_t refcount;
+  struct _upb_string *src;
+} upb_string;
+
+INLINE upb_strlen_t upb_string_bytelen(upb_string *str) { return str->byte_len; }
+INLINE const char *upb_string_getrobuf(upb_string *str) { return str->ptr; }
+
+upb_string *upb_string_getref(upb_string *str);
+void upb_string_unref(upb_string *str);
+upb_string *upb_string_tryrecycle(upb_string *str);
+
+// The three options for creating a string.
+char *upb_string_getrwbuf(upb_string *str, upb_strlen_t byte_len);
+void upb_string_substr(upb_string *str, upb_string *target_str,
+                       upb_strlen_t start, upb_strlen_t byte_len);
+void upb_string_refexternal(upb_string *str, char *ptr, upb_strlen_t len);
+