EEP-23: Allow variables in fun M:F/A

Currently, the external fun syntax "fun M:F/A" only supports
literals. That is, "fun lists:reverse/1" is allowed but not
"fun M:F/A".

In many real-life situations, some or all of M, F, A are
not known until run-time, and one is forced to either use
the undocumented erlang:make_fun/3 BIF or to use a
"tuple fun" (which is deprecated).

EEP-23 suggests that the parser (erl_parse) should immediately
transform "fun M:F/A" to "erlang:make_fun(M, F, A)". We have
not followed that approach in this implementation, because we
want the abstract code to mirror the source code as closely
as possible, and we also consider erlang:make_fun/3 to
be an implementation detail that we might want to remove in
the future.

Instead, we will change the abstract format for "fun M:F/A" (in a way
that is not backwards compatible), and while we are at it, we will
move the translation from "fun M:F/A" to "erlang:make_fun(M, F, A)"
from sys_pre_expand down to the v3_core pass. We will also update
the debugger and xref to use the new format.

We did consider making the abstract format backward compatible if
no variables were used in the fun, but decided against it. Keeping
it backward compatible would mean that there would be different
abstract formats for the no-variable and variable case, and tools
would have to handle both formats, probably forever.

Reference: http://www.erlang.org/eeps/eep-0023.html

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