Organize public and internal parts of memory management (#1539)

* Introduced new 'jmem.h' for public part.
 * Removed obsolete headers

JerryScript-DCO-1.0-Signed-off-by: László Langó llango.u-szeged@partner.samsung.com

jerry-core/jmem/jmem-allocator.c

@@ -18,9 +18,7 @@
  */
 
 #include "jcontext.h"
-#include "jmem-allocator.h"
-#include "jmem-heap.h"
-#include "jmem-poolman.h"
+#include "jmem.h"
 #include "jrt-libc-includes.h"
 
 #define JMEM_ALLOCATOR_INTERNAL