Erlang/OTP 20 warns if the stacktrace is read outside of a
catch/rescue. This commit mirrors this behaviour by consistently
warning on `System.stacktrace/0` being used outside of a
catch/rescue.
Erlang/OTP 21 warns whenever System.stacktrace/:erlang.get_stacktrace
are used. Therefore we need to promote the usage of `__STACKTRACE__`
and make sure to conditionally compile it according to the OTP version.
This requires changes to the Exception normalization mechanism
so we compute `__STACKTRACE__` only when strictly required.
In future Elixir releases, `System.stacktrace/0` will warn when
used even inside catch/rescue.