Replace function with func

[esoterra]

This implements #163 changing the function WIT keyword to be func.

[alexcrichton]

Thanks for this! I'll note though that I tagged #163 with syntax because I think we need to probably confirm we want to deviate from core wasm's and the component model's precedence of "func" with "function" but I don't think it's otherwise necessarily a clear-cut decision that we should migrate to "func". I think it would be good to get some feedback from other *.wit stakeholders and see what their thoughts are on a change like this as well.

[esoterra]

We'd love to start having those conversations. We're not dead-set on implementing this change, we just want to settle this question sooner than later because switching will only get harder.

(created a Zulip thread here to collect feedback)

[peterhuene]

While I personally prefer func based on the reasoning that it corresponds to the underlying WebAssembly proposal (also less typing...), I don't have a strong opinion on the particular token we use; however, I strongly agree that this needs to be decided upon by the stakeholders (open question: how do we identify the stakeholders?) sooner than later to avoid having a larger wit corpus in the wild to update.

[lukewagner]

Coincidentally, I was just having a chat with a number of distinct wit-bindgen users and, taking a quick poll of the room, essentially everyone said they preferred func to function (but were "fine" with function and hence hadn't cared enough to raise the issue). Personally, I think the argument that function is the only token that doesn't line up directly with the component model is pretty compelling.

[esoterra]

On a related note, the other thing that stands out is float32/float64 which is consistent between wit-bindgen and the component model, but both disagree with the WebAssembly spec which uses f32/f64. What was the rationale for that split?

[peterhuene]

@Kylebrown9 See Luke's prior comment (and the discussion that ensued) as to why there's a discrepancy between f32/f64 and float32/float64.

[alexcrichton]

Ok sounds like there's no major objections to this, and personally I find the symmetry between core wasm and the component model with the *.wit syntax pretty compelling. For that I'm going to merge, thanks @Kylebrown9 for doing this refactor!