OTP-28 readiness (part 1)

[ferd]

Covers portions of #2936, namely the bits about Dialyzer checks for xrl and yrl compilers, and for Common Test and EUnit.

Dependencies will need to be done in a different pull request.

[tsloughter]

Was changing append to maybe_append because it was broken and dialyzer caught it now or did another functions return type change?

[ferd]

Was changing append to maybe_append because it was broken and dialyzer caught it now or did another functions return type change?

Basically we used GoodResult | {error, Reason} as a return type internally, but GoodResult is a dict(), which is an opaque type, so any matching on it is raising opacity red flags. The append/2 function wrapped error as passthrough and assumed non-error results could be appended safely. lists:append/2 is called elsewhere in the module.

So to respect opacity constraints, I wrapped internal to uses {ok, GoodResult} | {error, Reason} all the way through, which lets us properly avoid opacity issues. I rename append to maybe_append to better represent its role as "append but with error handling".

Specifically, I think Dialyzer got better at catching some stuff it ignored before?