v3.6.0

CHANGES:

• A [%%cstruct type ...] declaration generates many values that
  are potentially unused. The code generator in ppx_cstruct now
  guarantees that there will be no more "unused value" (warning 32)
  statements from use of the ppx form. (#228 @emillon)
• Actually run the ppx tests instead of just building them.
  (#227 @emillon to fix #226 from @XVilka)