SDSL 2.2 (vgteam fork)

This is the initial release of the vgteam fork of SDSL. The baseline is simongog@c32874c (2019-12-10) that is 137 commits past the last official release 2.1.1.

Major changes

• SDSL now uses C++14.
• sd_vector improvements: predecessor/successor queries and an iterator over the set bits.

Fixes

• Most compiler warnings are gone.
• select_support_mcl::init_fast works correctly in some edge cases (simongog#402).
• Use csa_wt::select_bwt in suffix_array_helper.hpp (simongog#426).
• sd_vector_builder no longer fails silently (simongog#430).
• All tests pass (simongog#432).
• Use data from the UChile mirror of Pizza & Chili for benchmarks (simongog#434).
• Copyable int_vector_buffer iterators (simongog#436).