Revise some integer sizes.

size_t at the public API, uint8_t on the SSL structs since everything
fits in there comfortably.

Change-Id: I837c3b21e04e03dfb957c1a3e6770300d0b49c0b
Reviewed-on: https://boringssl-review.googlesource.com/12638
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/t1_lib.c

diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index 474f50a..3530ff5 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -3569,7 +3569,8 @@
     return -1;
   }
 
-  ssl->s3->new_session->original_handshake_hash_len = digest_len;
+  assert(sizeof(ssl->s3->new_session->original_handshake_hash) < 256);
+  ssl->s3->new_session->original_handshake_hash_len = (uint8_t)digest_len;
 
   return 1;
 }