add clone / insert methods

Summary:
cursor.clone() will clone a length of the chain.  insert(std::move(buf)) will splice in a length of chain
I want this to change thrift2 binary type to IOBuf: we will clone() the network data for zero-copy userspace data, and insert() it if the return value is a IOBuf.

Test Plan: added unittest

Reviewed By: afrind@fb.com

FB internal diff: D632073