Get at the length field of sha-512

This lets us take the hash of old-world strings with a bunch of trailing
zero bytes.

arvo/hoon.hoon

@@ -3519,11 +3519,14 @@
     [[256 d] $(c d, b (sub b 256))]
   --
 ++  shaz                                                ::  sha-512
-  ~/  %shaz
   |=  ruz=@  ^-  @
+  (shal [(met 3 ruz) ruz])
+++  shal                                                ::  sha-512 with length
+  ~/  %shal
+  |=  [len=@ ruz=@]  ^-  @
   =+  [few==>(fe .(a 6)) wac=|=([a=@ b=@] (cut 6 [a 1] b))]
   =+  [sum=sum.few ror=ror.few net=net.few inv=inv.few]
-  =+  ral=(lsh 0 3 (met 3 ruz))
+  =+  ral=(lsh 0 3 len)
   =+  ^=  ful
       %+  can  0
       :~  [ral ruz]