Experiment with supporting full-body query functions

[hauleth]

For now cond statements are supported, but it can uncleanly fail if there is unsupported construct in the query function body.

Up to discussion:

• maybe support some of the cases (with the constants as the match patters) as PostgreSQL supports such construct
• if and unless, but it will make them work slightly different from the Elixir one, as PostgreSQL is statically typed, so there is no way to simulate it for non-bool condition
• variables assignment? I am not even sure how that could work at all, but if someone have any idea, then I will be open

Things needed to be done before merging:

• Tests, extensive test suite
• Clean handling of invalid function body

---

Goal of this PR is to add support for more complex query functions, for example support conditionals like:

defq clamp(val) do
  cond do
    val < 0.0 -> 0.0
    val > 1.0 -> 1.0
    true -> true
  end
end

Which then can be used in the query just as you would expect:

from entry in "entries",
  select: %{clamped: clamp(entry.value)}