Figure out how to separate reporters-standard from core

[malsyned]

Currently, reporters-standard is included in the core/directory. Technically, it is not part of the core. The core requires some reporter, but the XML reporter can be included and the standard reporters left out. This may be desirable on small embedded components.

The convention I'm trying to keep to is that each subdirectory of src is either:

• To have all of its C files built into a project together
• To have exactly one C file built into a project

Reporters are slightly different, because the CLI expects both, but only one of them is technically required.

[malsyned]

I just bit the bullet and put them all in a reporters/ folder. The typical use cases will still be to either include only one or to include them all.