Write support for squashfs archives

[Gottox]

Is your feature request related to a problem? Please describe.

libsqsh was intended to be read only. There are already libraries and tools out there that do a pretty good job in creating squashfs images. The reason why libsqsh was even started was, that there is no decent non-GPL implementation which is a showstopper for many use cases. Now that the reading part is in decent shape we're facing new issues: Creating new test cases is often clumsy and requires deep knowledge of the squashfs file system. Having a way to tailor custom squashfs archives for test cases would be awesome.

Describe the solution you'd like

• have an independent builder API to create squashfs images and write them to a FILE* (seekable) pointer.
• The code sharing between the reader and the writer API should be minimal. There are certain things that could be reused (like the SqshMapper APIs and the on-disk data structures) but there are certain runtime characteristics that benefit from the fact, that most of the data in libsqsh are immutable.

Describe alternatives you've considered

Not do it at all and rely on squashfs-tools-ng to generate test cases on the fly.