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/private/parser.h

@@ -83,4 +83,8 @@ xmlParserNsUpdateSax(xmlParserCtxtPtr ctxt, const xmlChar *prefix,
 XML_HIDDEN void *
 xmlParserNsLookupSax(xmlParserCtxtPtr ctxt, const xmlChar *prefix);
 
+XML_HIDDEN xmlParserInputPtr
+xmlNewInputPush(xmlParserCtxtPtr ctxt, const char *url,
+                const char *chunk, int size, const char *encoding);
+
 #endif /* XML_PARSER_H_PRIVATE__ */