Variable Length Gap Pattern Matching

[olydis]

Note: this branch is based on https://github.com/simongog/sdsl-lite/tree/new_wt and had significant differences to sdsl-lite/master.

I manually tried to merge vlg_matching towards sdsl-lite master, yet without breaking vlg_matching and new_wt's "ys_in_xrange" example.

In case the ys_in_xrange-functionality is not desired in the master-branch, I can further reduce this PR.

[tb-robot]

Can one of the admins verify this patch?