Add function field serialisation / default helpers

This adds helper functions for (de)serialisations and default
generation of function input/output/error fields to `$MOD_piqi_rpc.erl`.

The rationale is that, unlike types, the actual function (in particular, its
name) is something which exists only in `$MOD_piqi_rpc.erl`, so it makes sense to
provide these helpers and abstract from the actual type definitions the
function fields.

This also allows to slightly simplify some of the other generated code.
Merge pull request #3 from yfyf/rpc_func_helpers

Add function field serialisation / default helpers