Make application:stop/1 callback optional

[josevalim]

The majority of the times it simply returns ok.
In Erlang/OTP itself, only ssl and snmp implemented it.

Note we didn't have to change the implementation
because the callback is always invoked as
catch App:stop(Arg), which is also how other optional
callbacks, such as prep_stop/1 are handled.

[github-actions]

CT Test Results

   13 files    313 suites   3h 11m 37s ⏱️
5 573 tests 5 085 ✅ 486 💤 2 ❌
8 084 runs  7 445 ✅ 637 💤 2 ❌

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Results for commit b1e516b.

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// Erlang/OTP Github Action Bot

[josevalim]

CI is failing but it may be a cache issue (other applications being recompiled before kernel?). Or let me know if you would prefer me to break this in two PRs (one that changes application.erl, another that removes its usage).

[garazdawi]

@josevalim CI fails because the primary bootstrap needs to be updated. I pushed a commit that does that.

I'll leave deciding if we want this or not to next week.

[essen]

A lot of the time the start function only calls my_sup:start_link as well. If there's nothing special to do for starting or stopping the application, then perhaps we can skip that module altogether?

That said, that module is typically filled by templates or copy pasting, so it being optional, or the stop callback being optional, doesn't really change much.

[max-au]

Speaking of which, there is also *.app file that specifies which module contains the actual application behaviour. Maybe it's better to support it this way:

{application, myapp,
 [{description, "Description of myapp"},
  {vsn, "1.0.0"},
  {supervisor, {myapp_sup, []}},

Meaning, instead of {mod, myapp}, it'd have {supervisor, myapp_sup}.

The more I think about it, the more I like it. Less code - less space for bugs.