xalloc impl...maybe. needs testing.

include/nickel/xalloc.h

@@ -13,7 +13,6 @@ extern "C" {
 #endif
 
 void* ni_xmalloc(size_t size);
-void ni_free(void* ptr);
 
 void* ni_xcalloc(size_t nmemb,size_t size);
 void* ni_xrealloc(void* ptr, size_t size);