eolian_cxx: Add key/value to function_def

eolian_cxx: Add key/value to function_def

Summary:
This commit adds explicit eolian key/value/return info to function_def. The
original parameter/return types are kept for compatibility.

parameter/return -> Mimics the behavior of the generated C API (i.e.
single-valued getters have their lone value translated to be their
return type)

explicit_return -> The actual return value for that block in the EO
file.
keys/values -> The values for respectively key and values blocks.

This should help working with properties in the generators instead of
fumbling with paremeter/return directly

Fixes T8431

Test Plan: extra tests in the diff

Reviewers: felipealmeida, brunobelo, segfaultxavi, YOhoho

Reviewed By: brunobelo

Subscribers: cedric, #reviewers, #committers, woohyun

Tags: #efl

Maniphest Tasks: T8431

Differential Revision: https://phab.enlightenment.org/D10494