Release 0.2

Features added:

• grid-info: More functions and aliases; some function renames.
• Non-builtins: rotate_left(), rotate_right(), log2().
• #88: Simple integer ranges, and special ranges for warp, block and grid-stride iteration, instead of calling at_grid_stride, at_block_stride etc.

Features removed:

• strf-based stringstreams: They're currently broken. They may make a comeback in a future release.

Other noteworhty changes:

• #91: Dropped the use of extra sub-namespaces: grid_info and collaborative
• #72: Reductions no longer take a NeutralValue template parameter.
• #74: Fixed nanosecond sleep.
• #84: C++17 compatibility (but this is still not covered by unit tests)