More function pushdown code

- Push down CONCAT()
- Don't check for mutable functions - we are explicitly
  whitelisting functions which can be pushed down
  (need to double-check if that's a valid approach)
Push down POSITION() / STRPOS()
Release 0.1.6

Conclude function/operator pushdowns for now, though I am sure
we will revisit this at some point to fix whatever alarming bugs
and misconceptions are lurking here.