Improve LZMA decoder using 7zip logics for modern dotnet

- Add opt-in Parallel decoding
- Add a new ASM-like decoder
- Add Unit tests for Parallel decoding

docs/API.md

@@ -332,6 +332,10 @@ var hinted = ReaderOptions.ForExternalStream.WithExtensionHint("tar.gz");
 // Increase for non-seekable streams with large detection probes, such as SFX RAR
 var buffered = ReaderOptions.ForExternalStream.WithRewindableBufferSize(1_048_576);
 
+// Opt in to formats' optional parallel decode (e.g. 7-Zip's automatic parallel LZMA2
+// solid-folder decode); default is sequential decoding
+var parallel = ReaderOptions.ForExternalStream.WithEnableParallelism(true);
+
 // Extraction presets
 var safeOptions = ExtractionOptions.SafeExtract;  // No overwrite
 var flatOptions = ExtractionOptions.FlatExtract;  // No directory structure
@@ -362,6 +366,7 @@ var options = new ReaderOptions
     DisableCheckIncomplete = false,
     BufferSize = 81920,
     RewindableBufferSize = 1_048_576,
+    EnableParallelism = false,
 };
 
 var extractionOptions = new ExtractionOptions
@@ -444,6 +449,8 @@ var options = new WriterOptions(CompressionType.Deflate)
 archive.SaveTo("output.zip", options);
 ```
 
+`WriterOptions.EnableParallelism` (also available on `ZipWriterOptions`, `TarWriterOptions`, `GZipWriterOptions`, and `SevenZipWriterOptions`) opts in to a format's optional parallel encode. No writer currently implements parallel encoding, so this is a no-op today; it is reserved for future use and mirrors `ReaderOptions.EnableParallelism`.
+
 ### Extraction behavior
 
 ```csharp