Adding a new buf module for buffers

This also add converter functions between xmlBuf and xmlBuffer
* buf.c buf.h: the old xmlBuffer routines but modified for size_t
  and using xmlBuf instead of xmlBuffer
* Makefile.am: add the 2 new files
* include/libxml/xmlerror.h: add an entry for the new module
* include/libxml/tree.h: expose the xmlBufPtr type but not the
  structure which stay private
diff --git a/buf.h b/buf.h
new file mode 100644
index 0000000..4d3afd0
--- /dev/null
+++ b/buf.h
@@ -0,0 +1,64 @@
+/*
+ * buf.h: Internal Interfaces for memory buffers in libxml2
+ *
+ * See Copyright for the status of this software.
+ *
+ * daniel@veillard.com
+ */
+
+#ifndef __XML_BUF_H__
+#define __XML_BUF_H__
+
+#include <libxml/tree.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+xmlBufPtr xmlBufCreate(void);
+xmlBufPtr xmlBufCreateSize(size_t size);
+xmlBufPtr xmlBufCreateStatic(void *mem, size_t size);
+
+int xmlBufSetAllocationScheme(xmlBufPtr buf,
+                              xmlBufferAllocationScheme scheme);
+int xmlBufGetAllocationScheme(xmlBufPtr buf);
+
+void xmlBufFree(xmlBufPtr buf);
+void xmlBufEmpty(xmlBufPtr buf);
+
+size_t xmlBufShrink(xmlBufPtr buf, size_t len);
+int xmlBufGrow(xmlBufPtr buf, int len);
+int xmlBufInflate(xmlBufPtr buf, size_t len);
+int xmlBufResize(xmlBufPtr buf, size_t len);
+
+int xmlBufAdd(xmlBufPtr buf, const xmlChar *str, int len);
+int xmlBufAddHead(xmlBufPtr buf, const xmlChar *str, int len);
+int xmlBufCat(xmlBufPtr buf, const xmlChar *str);
+int xmlBufCCat(xmlBufPtr buf, const char *str);
+int xmlBufWriteCHAR(xmlBufPtr buf, const xmlChar *string);
+int xmlBufWriteChar(xmlBufPtr buf, const char *string);
+int xmlBufWriteQuotedString(xmlBufPtr buf, const xmlChar *string);
+
+size_t xmlBufAvail(xmlBufPtr buf);
+size_t xmlBufLength(xmlBufPtr buf);
+size_t xmlBufUse(xmlBufPtr buf);
+int xmlBufIsEmpty(xmlBufPtr buf);
+int xmlBufAddLen(xmlBufPtr buf, size_t len);
+int xmlBufErase(xmlBufPtr buf, size_t len);
+
+xmlChar * xmlBufContent(const xmlBufPtr buf);
+xmlChar * xmlBufEnd(const xmlBufPtr buf);
+
+xmlChar * xmlBufDetach(xmlBufPtr buf);
+
+size_t xmlBufDump(FILE *file, xmlBufPtr buf);
+
+xmlBufPtr xmlBufFromBuffer(xmlBufferPtr buffer);
+xmlBufferPtr xmlBufBackToBuffer(xmlBufPtr buf);
+int xmlBufMergeBuffer(xmlBufPtr buf, xmlBufferPtr buffer);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* __XML_BUF_H__ */
+