parser: New input API

Provide a new set of functions to create xmlParserInputs. These can be
used for the document entity or from external entity loaders.

- Don't require xmlParserInputBuffer.
- All functions take a base URI.
- All functions take an encoding as string.
- xmlNewInputURL also takes a public ID.
- xmlNewInputMemory takes a size_t.
- Optimization hints for memory buffers.

Improve documentation.

Only call xmlInitParser before allocating a new parser context.

Call xmlCtxtUseOptions as early as possible.

include/libxml/HTMLparser.h

@@ -246,6 +246,8 @@ XMLPUBFUN htmlDocPtr
 					 const char *URL,
 					 const char *encoding,
 					 int options);
+XMLPUBFUN htmlDocPtr
+		htmlCtxtParseDocument	(htmlParserCtxtPtr ctxt);
 XMLPUBFUN htmlDocPtr
 		htmlCtxtReadDoc		(xmlParserCtxtPtr ctxt,
 					 const xmlChar *cur,