C-CUSTOM-TYPE: Cover return types + discourage tuples.

[anforowicz]

PTAL?

This PR is an attempt to upstream some Chromium-specific guidelines from https://source.chromium.org/chromium/chromium/src/+/main:docs/rust/api_design.md;l=56-66;drc=e213d50dfa608aba386b151f831d6e25f849bcac.

I hope that covering not only parameter types, but also return types is mostly uncontroversial.

I think that discouraging the use of tuples in public APIs matches the spirit of the rule. I understand that sometimes the meaning of tuples is obvious from the context - e.g. it seems totally fine to return a tuple from unzip. But I think that the situation here is similar to a bool return type - it may be sometimes okay to return a bool (e.g. from is_empty or from is_small or is_round), but one should still consider using a more specific parameter and/or return type if possible.

The fact that bool is okay in many is_foo functions makes me wonder if this PR should instead be adding a new rule to the guidelines. But I think that editing the C-CUSTOM-TYPE rule is still reasonable and keeps things simple. WDYT?

/cc @DKLoehr, @zetafunction, @Manishearth