v3.0.0

• billy.Filesystem has been split in several dedicated interfaces, also the test suite has been divided in several suites.
• subdirfs & tmpfs: converted to helpers, this fs are not anymore handled as fs, since they are not.
• memfs & osfs: based on helper chroot, simplifying a lot the code and make it less error prone.
• helper.Chroot has being covered by a dedicated test suite to make it easier detected issues.