Fix reverting of implicit funs

Implicit funs parts in plain AST are no longer in concrete form since
Erlang/OTP R15.
Fix transformation of implicit funs in igor

Function erl_syntax_lib:analyze_implicit_fun/1 should not be called on
implicit external funs, as their parts can contain variables instead of
atoms and integers.
Remove a useless function clause in erl_syntax_lib
Prepare release