Optmize vertex and edge builder functions

[rafsun42]

Changes:
- Added the agtype_raw module, which contains helper functions to
build agtype directly without building an agtype_value first
- Optimize _agtype_build_vertex and _agtype_build_edge functions

The agtype_raw module:
Inserting a composite agtype (i.e. object) into another agtype
(i.e. as an array element) requires the first agtype to be deserialized
into agtype_value. Then, the agtype_value to be serialized back into
the second agtype.

This module provides functions that can perform such insertion without
deserializing first. It is meant to speed up queries that does
deserialization-serialization back and forth involving deeply nested
agtype objects.