Update join

Added tests for `join`.
Join key inference was failing due to the change to symbols, so I fixed that.
Join key inference was picking one at random when there were multiple common colnames -- I set it to throw if there's more than one as it's a little dangerous in my experience, but I can change that if people don't like it.
I added join kinds `:semi` and `:anti`.


Collateral: The test file I copied had tabs, so I cleared tabs out of all test files. Updated `test/RDA.jl` to run from anywhere like Simon did for `test/io.jl`.