Bugfix for precondition_context test failures.

Previously every riak_dt type simply called riak_dt:to_binary/1 for
serialization, and before 2.0.0 optimizations, was an entire copy of the
datatype. This meant that the precondition context was handled through
the type's to_binary/1 function. The 2.0.0 optimizations use only a VV
as the context, however, so we don't need to serialize via the type's
to_binary/1 call, we can call riak_dt_vclock:to_binary/1 directly.

This bug was uncovered by riak_dt#110, which changes how the sets and
maps are manipulated before serialization.
Map now sorts keys in its value output.
Fix broken nesting
Merge pull request #1063 from basho/bugfix/sdc/crdt-context-binary

Bugfix for precondition_context test failures.

Reviewed-by: russelldb