Add canAdvance to cursor and tests

Add canAdvance to cursor and tests

Summary:
Determine if the cursor can advance N bytes. This is useful if
applications want to check before reading so an exception isn't thrown.
It tries to walk the minimal amount of links needed in the chain.

I had a task that could have used this, though caching totalLength and
macro magic ended up being the implementation chosen. I think this just
adds to the cursor API.

Reviewed By: djwatson

Differential Revision: D2728498

fb-gh-sync-id: 8657653b82a48828cccab143653dc169ef715702