This version of Pairs.Add is simpler and [more
performant](https://gist.github.com/gbjk/06a1fc1832d04ee41213ca518938cf74)

Behavioural difference: If there's nothing to add, the same slice is
returned unaltered. This seems like good sauce

Now only the assetPairs relevant to the subscription are in the context

We want to trim before checking for "AssetSeparator vs All" because a
template should be allowed to reuse a range template and generate just one trailing AssetSeparator
whilst using a specific Asset

Turns out that contary to the documentation, kucoin supports batching of
all symbols and currencies

This reduces the complexity of error checking to just be "do we get the
correct numbers".

Fixes Asset.All with only one asset erroring on xpandPairs, because we
trimmed the only asset separator, and then errored that we're not
xpanding Assets and the asset on the sub is asset.All

This use-case conflicted with commit 6bbd546, which required:
```
Subscriptions: Trim AssetSeparator early

We want to trim before checking for "AssetSeparator vs All" because a
template should be allowed to reuse a range template and generate just one trailing AssetSeparator
whilst using a specific Asset
```

Now we set up the assets earlier, and we remove the check for xpandAssets, since the number of asset lines matching is all that matters.

I've removed the asset tests for this, but they were correctly erroring
on the number of asset lines instead.

Everything hits coverage, as well.