Generalized factories to readers and writers.

[vpenades]

What's changed:

• Created a new entry point for registering factories in a new folder called Factories.Factory (for lack of a better name)
• I've moved the ArchiveFactories (ZipArchiveFactory, etc) to the new Factories folder
• Added new interfaces IReaderFactory and IWriterFactory

With these new interfaces in place, I've been able to clean up some code; the idea is to eventually get rid of all the if zip else if gzip else if rar else if tar... spaghetti blocks. This will simplify adding new formats because everything will be centralized in a single factory class.

Supporting the TAR.GZ and variants has been specially tricky, I wanted to retain as much of the original logic as possible to avoid breaking something, so I had to create an internal overridable method Factory.TryOpenReader that handles the internal RewindableStream logic. Fortunately, new formats will not needs this.

From now on, adding a new archive format is about extending "Factory" class, and implement any of IArchiveFactory, IReaderFactory and IWriterFactory on it.

SevenZipFactory is an example of a factory implementing only IArchiveFactory but not IReaderFactory. To some degree, these interfaces work as decorators, so an archive format only needs to implement the interfaces it can support.

Future: there's still some spaghettis around, like getting multipart files, or creating a writeable archive... I think these could be done by adding additional decoration interfaces.

[vpenades]

I think I've covered all the functions that depended on explicit case per case formats, and now everything is generalized into the factories. On my side I think this PR is ready for review. Let me know if there's something you don't like and want it changed/removed.

[adamhathcock]

Minor question but looks good!

[adamhathcock]

more extensions for LZip/BZip2 types? tbz or tlz? those are guesses.

[vpenades]

I've just added the extensions reported by wikipedia, didn't know tar had so many flavours!

I was tempted to add the "tar.*" variants, but I am aware some APIs don't handle well extensions with dots and can produce misleading results.

[adamhathcock]

nah, we'll let the user handle nested extensions...

some of these we don't handle though... LZMA we could but XZ, zstd we don't *yet and not sure what compress is

[vpenades]

I've commented these extensions, I take it tZ and taZ are supported?