Add guidance on functions and tasks

[GregAC]

Fixes #28

I've gone beyond the scope of what we've discussed in #28 in terms of rules/recommendations here. I am happy for these to be challenged just things that came up whilst I was writing it that I felt needing addressing. A brief list of what I've added on top if the #28 discussion is:

• No tasks, functions only
• Functions in packages or inside modules (not just outside a module in a non package .sv)
• Recommendations around use of return result vs function_name = result
• Specifying all local variables must get assigned on all code paths

[imphil]

@GregAC Looks like this PR is almost there, can you incorporate the last suggestions from Scott?

[GregAC]

Yup don't worry haven't forgotten about it.

I think the only real point of discussion (beyond corrections, small improvements/additions etc) was how restrictive we want to be on what functions can take as arguments.

I'd quite purposefully gone for a function as a stand-in for a block of combinational logic style and said input only via arguments with only one output.

Multiple outputs are useful but of course still possible, you just need to concatenate things or use a struct as a return type. In my experience people generally don't do super-complex things with functions in RTL so I don't think disallowing output arguments will cause major issues.

I'll update the style guide today assuming we're happy with that idea but give the other commenters a chance to reply to continue the discussion if they still disagree with that style.

[GregAC]

Apologies for the slow updates, have now incorporated all of the feedback, though a couple of places where I disagreed, see the comments.

Thanks for the reviews @sriyerg @sjgitty @imphil @msfschaffner

[eunchan]

I also like this approach. Thanks! LGTM

[GregAC]

Hitting the merge button on this as I think everyone's more or less happy with it. If there is something people disagree with feel free to continue the discussion, we can always do another PR to change things.