Generate op_enum.h during build rather than a separate step

[Dewb]

What does this PR do?

This is an illustration of a potential approach to #340.
The changes:

• runs op_enum.py to generate op_enum.h during the build process
• moves all generated code files to /src/gen
• moves the codegen build steps common to the module, test, and simulator Makefiles to codegen.mk

Provide links to any related discussion on lines.

How should this be manually tested?

Any background context you want to provide?

If the related Github issues aren't referenced in your commits, please link to them here.

I have,

• updated CHANGELOG.md and whats_new.md
• (n/a) updated the documentation
• (n/a) updated help_mode.c (if applicable)