Move previewer task and channel into functions

[danielsoutar]

Hi,

Hope this MR makes ReTest a little more accessible to newcomers. This MR is pretty much done, maybe bar one or two points to clarify (specifically what align_overflow and maxidw are for in the ReTest.jl), although I am failing about 7 or so tests. These are:

e: Test Failed at /path/to/ReTest.jl/test/runtests.jl:988
z4: Test Failed at /path/to/ReTest.jl/test/runtests.jl:1007
z4: Test Failed at /path/to/ReTest.jl/test/runtests.jl:1007
has fails: Test Failed at /path/to/ReTest.jl/test/runtests.jl:1251
a2: Test Failed at /path/to/ReTest.jl/test/runtests.jl:1300
b1: Test Failed at /path/to/ReTest.jl/test/runtests.jl:1304
c1: Test Failed at /path/to/ReTest.jl/test/runtests.jl:1308

Some of these are under a header named Main.FailingLoops, so I'm not sure if these are meant to fail or not?

This is not meant to introduce any new functionality - just better structure and documentation for myself and others' benefit when reading the code.

EDIT: All the tests above have failing conditions, so this is fine. As an aside, it might be helpful to separate these into a 'failing tests' file so that it's easier for someone to know that this is expected behavior.

[danielsoutar]

For those that can write/merge to this repo - I have other code I would like to split out into functions. Let me know if you'd prefer to consolidate those into this branch, or if you'd rather have smaller and more focused MRs instead.

[rfourquet]

Thanks that looks great! The retest code has indeed grown way too big and I've wanted to see it split into smaller parts for a while. As a side note, I also believe that there might be a better design for handling the different tasks (previewer, printer, worker), but haven't gone around it yet. But this PR is definitely a very good step. Give me a few days to review. And yes, I would definitely "rather have smaller and more focused MRs instead."
I'm super happy to see contributions to this package!