LibAudio: Move WAV sample reading and conversion into own helpers

This completely removes WavLoader's dependency on LegacyBuffer: We
directly create the result sample container and write into it. I took
this opportunity to rewrite most of the sample reading functions as a
single templated function, which combined with the better error handling
makes this "ported" code super concise.

Userland/Libraries/LibAudio/WavLoader.h

@@ -7,12 +7,12 @@
 
 #pragma once
 
+#include <AK/FixedArray.h>
 #include <AK/OwnPtr.h>
 #include <AK/RefPtr.h>
 #include <AK/Span.h>
 #include <AK/String.h>
 #include <AK/StringView.h>
-#include <AK/WeakPtr.h>
 #include <LibAudio/Loader.h>
 #include <LibCore/File.h>
 #include <LibCore/Stream.h>
@@ -53,6 +53,10 @@ public:
 private:
     MaybeLoaderError parse_header();
 
+    LoaderSamples samples_from_pcm_data(Bytes const& data, size_t samples_to_read) const;
+    template<typename SampleReader>
+    MaybeLoaderError read_samples_from_stream(Core::Stream::Stream& stream, SampleReader read_sample, FixedArray<Sample>& samples) const;
+
     // This is only kept around for compatibility for now.
     RefPtr<Core::File> m_file;
     OwnPtr<Core::Stream::SeekableStream> m_stream;