Introduce the attribute -optional_callbacks in the context of behaviours
[syntax_tools] Let erl_syntax:concrete() accept the F/A syntax

Note: `arity_qualifier` nodes are recognized. This is to follow The
Erlang Parser when it comes to wild attributes: both {F, A} and F/A
are recognized, which makes it possible to turn wild attributes into
recognized attributes without at the same time making it impossible to
compile files using the new syntax with the old version of the Erlang
Compiler.
Update primary bootstrap

It's necessary to have behaviour_info(optional_callbacks) working
before building the rest of the primary bootstrap.
Update primary bootstrap