refactor(api): refactor more api functions to use arena return

Currently having two separate memory strategies for API return values is
a bit unnecessary, and mostly a consequence of converting the hot spot
cases which needed it first. But there is really no downside to using
arena everywhere (which implies also directly using strings which are
allocated earlier or even statically, without copy).

There only restriction is we need to know the size of arrays in advance,
but this info can often be passed on from some earlier stage if it is
missing.

This collects some "small" cases. The more complex stuff will get a PR
each.

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