read-cache*.h: move declarations for read-cache.c functions from cache.h

For the functions defined in read-cache.c, move their declarations from
cache.h to a new header, read-cache-ll.h.  Also move some related inline
functions from cache.h to read-cache.h.  The purpose of the
read-cache-ll.h/read-cache.h split is that about 70% of the sites don't
need the inline functions and the extra headers they include.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-hash-speed.c

@@ -1,5 +1,6 @@
 #include "test-tool.h"
 #include "cache.h"
+#include "hash-ll.h"
 
 #define NUM_SECONDS 3