Introduce xmlXPathSetErrorHandler allowing to set a structured error
handler for an XPath context.
Remove arguments from memory error handlers.
Use xmlRaiseMemoryError.
Remove TODO, STRANGE and CHECK_CTXT macros.
Remove remaining uses of xmlGenericError.
Fix many places where malloc failures aren't reported.
Rework XPath object cache to store free objects in a linked list to
avoid allocating an additional array. Remove some unneeded object pools.
