Remove naming requirement for std::pair/std::tuple #85

When converting C++ std::pair and length 2 std::tuple values to and from
capn'proto structs, no longer require the struct to have fields named "key" and
"value".  Instead always map the first pair/tuple element to the first struct
field, and the second element to the second field.

This allows anonymous std::pair and std::tuple types used in the C++ interface
to have names and custom Read/Build logic when they are serialized as
capn'proto messages.

For now only length-2 tuples are supported but this could be extended to
support converting c++ tuples of arbitrary length to capn'proto structs with
the same number of fields.