Fixes for Dialyzer

[paulo-ferraz-oliveira]

I start by removing all -dialyzer directives and then issuing fixes for the found issues.

I'll add self-review comments where appropriate, to ease review.

[paulo-ferraz-oliveira]

===> Error in dialyzing apps: Unknown dialyzer warning option: no_unknown

This is a bummer: I'ma fix CI so it only runs Dialyzer on a specific OTP version.

[paulo-ferraz-oliveira]

CI's failing while we wait for an hex_core release. Then we update rebar.config+rebar.lock and retry 😄

[paulo-ferraz-oliveira]

@starbelly, this is good to go. No more -dialyzer exceptions 😄