Bump to 0.12.3-dev
Ensure heredocs are allowed in calls, closes #1998
Guarantee nullary funs/macros are allowed in guards, closes #1997
Remove deprecated code
Remove @hidden in mix tasks

Instead, we can use the presence or abscence of @shortdoc
to specify if the task should be shown or not.
List iex -S mix at the bottom of mix help
Change precedence of <- to be the same as other generators
Add index reflection to private records

Example:
defrecordp :user, [:name, age]
user(:age) #=> 2