!str #16066 rename connect to via/to

* add missing implicit conversions for ~>
* cleaned up implicit conversions for ~>
* tests for all combinations when using ~>

Refs #16066
There is one disadvantage of moving the implicit builder to next step. Then the ~> operator can be used outside of the FlowGraph builder scope, i.e. just by importing FlowGraphImplicits._ at the top of the file. That usage is harmless, but still doesn't make sense.