AK: Rename span() to bytes() when appropriate.

I originally defined the bytes() method for the String class, because it
made it obvious that it's a span of bytes instead of span of characters.

This commit makes this more consistent by defining a bytes() method when
the type of the span is known to be u8.

Additionaly, the cast operator to Bytes is overloaded for ByteBuffer and
such.

Libraries/LibCrypto/Cipher/Cipher.h

@@ -65,7 +65,7 @@ public:
     virtual const ByteBuffer& data() const = 0;
 
     virtual void overwrite(const ReadonlyBytes&) = 0;
-    virtual void overwrite(const ByteBuffer& buffer) { overwrite(buffer.span()); }
+    virtual void overwrite(const ByteBuffer& buffer) { overwrite(buffer.bytes()); }
     virtual void overwrite(const u8* data, size_t size) { overwrite({ data, size }); }
 
     virtual void apply_initialization_vector(const u8* ivec) = 0;