Move Keccak code to its own folder and add an absorb API.

This is to prepare a Dilithium implementation.

Change-Id: I20581690c72837177823213fd2d3ad311614a660
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/63586
Reviewed-by: Bob Beck <bbe@google.com>
diff --git a/crypto/CMakeLists.txt b/crypto/CMakeLists.txt
index 56f9b90..3cde009 100644
--- a/crypto/CMakeLists.txt
+++ b/crypto/CMakeLists.txt
@@ -175,7 +175,7 @@
   ex_data.c
   hpke/hpke.c
   hrss/hrss.c
-  kyber/keccak.c
+  keccak/keccak.c
   kyber/kyber.c
   lhash/lhash.c
   mem.c