data/range_ptr: clean up, document more.
data/range_ptr: fix two bugs in policy support.

Comparison operators were defined incorrectly for any RangePtr using a
non-lax policy -- the overloads would not apply.

The asserting policy that shipped in range_ptr.h had a math error that
could produce slices referencing unsafe memory.
data/range_ptr: iteration support.
data/crc32: a CRC32 implementation.

This is pretty pedestrian -- perhaps the most interesting part is the
table generator, which is written constexpr and evaluated at compile
time.
Merge remote-tracking branch 'origin/master'