Rethink definition of variables in the result of a join

[krlmlr]

Simplify name generation and resolution of clashes:

1. Use variables, remove rhs variables for equi-joins (but not for generic joins)
2. Add suffixes to remaining variables of the same name in lhs and rhs
3. Combine variables: join variables, remaining lhs variables, remaining rhs variables
4. Apply tibble::set_tidy_names().

For dplyr, the result should be two lists that can be unquote-spliced into select(). The C++ side then only needs to know the number of join variables and return the result, For dbplyr, something similar to the current join_vars() implementation would be useful. Need to think about a data structure that supports both, and export the function that computes the join variables.

Useful for/part of #2240.

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