Bisect_ppx 2.8.0

Additions

• HTML: collapsible tree index for large projects, activated by --tree option (#396, Arvid Jakobsson).

Bugs fixed

• Don't instrument lazy values which are compiled as already forced (#398, reported by @Ngoguey42).
• Use Sys.backend_type to detect js_of_ocaml (#397, Pat Rondon).
• Text summary report: show files with 0 points as having 100% coverage (f3b9108).