This reverts commit 29e9f7f5.

* make the factory always return writers via shared_ptr
* add a PathWriter wrapper for opening a stream for the lifetime of a normal Writer
* wrap writer factory exceptions
* hide some writer factory implementation details in dteail:: sub namespace
* rename TokenWriter::create to create_stream_writer, add create_path_writer
* bump library version

Allow custom "ign" tag equivalent in tagsets, fix tests accordingly as now a valid ign tag is required

Reader factory. There is some code duplication with the writer factory as well as among the individual Readers, but overall it is now easier to support more input formats.

Fast and drity (regex-based) XCES parser based on the one in Pantera, noticeably faster but prone to breakage on even slightly freeform xml files

move count_bits_set to pwrutisl, put lowest_bit alias there, implement Tagset::tag_size and Tagset::tag_is_singular

Libcorpus2 now contains the base strructures (tokens etc), their I/O and related classes.
Libpwrutils (name TBD) contains basic building blocks common to all tagger-related libraries
Tagset-tool only needs libcorpus, so was brough here as well.