Codegen cleanup

[gucio321]

Hi there!
First of all, I'm sorry for this being so large PR. The situation got a bit out of control. 😄
I'll try to briefly point what was changed here but forgive if I've missed something
In general I go rid of this large generateGoFuncs function - it is now of human-readable size

• split gengo.go into a several files (one for each "datatype") - the further steps are for gengo_funcs.go - it is the most complex one
• create an internal-state-type for function generator. It holds shared variable such as StringBuilder
• split generateGoFuncs function into several smaller steps
• simplify function generation (redesign code; remove "duplicated" code)
• add some comments

Let me know what do you think about these changes as well as feel free to suggest another one!
IMO the code is easier to read now but let me know if it isn't ;-)

[AllenDang]

@gucio321
First of all, big thanks for your effort! Really appreciate your contribute for this repo.
I have a suggestion, do the business first, optimize later. The purpose for this repo and giu is to empower the ability of GUI creation for Golang, so let's spend our effort on make giu more usable, stable and powerful, and drive the optimization from giu to cimgui-go.
What's your opinion?

[AllenDang]

LGTM

[gucio321]

Sure! I'm going to focus on giu as soon as cimgui-go shares all the necessary functions.
To do so however, we need to have this codegen script easy to fix and extend.