AK+Everywhere: Change AK::fill_with_random to accept a Bytes object

Rather than the very C-like API we currently have, accepting a void* and
a length, let's take a Bytes object instead. In almost all existing
cases, the compiler figures out the length.

Userland/Libraries/LibCrypto/NumberTheory/ModularFunctions.cpp

@@ -168,7 +168,7 @@ UnsignedBigInteger random_number(UnsignedBigInteger const& min, UnsignedBigInteg
     auto buffer = ByteBuffer::create_uninitialized(size).release_value_but_fixme_should_propagate_errors(); // FIXME: Handle possible OOM situation.
     auto* buf = buffer.data();
 
-    fill_with_random(buf, size);
+    fill_with_random(buffer);
     UnsignedBigInteger random { buf, size };
     // At this point, `random` is a large number, in the range [0, 256^size).
     // To get down to the actual range, we could just compute random % range.