A mistake I've repeatedly made is along these lines:
```c++
auto nread = TRY(source_file->read(buffer));
TRY(destination_file->write(buffer));
```
It's a little clunky to have to create a Bytes or StringView from the
buffer's data pointer and the nread, and easy to forget and just use
the buffer. So, this patch changes the read() function to return a
Bytes of the data that were just read.
The other read_foo() methods will be modified in the same way in
subsequent commits.
Fixes#13687
MemoryStream is the Core::Stream API's streamlike access to a chunk of
memory, mimicking AK::DuplexMemoryStream. The implementation is very
similar, except that no APIs except the SeekableStream operations
currently exist. This will be fine for the first users and can be
expanded upon later.
