Add support to functions within `@slice()`

[kdpsingh]

Right now, this doesn't work as a way to shuffle the rows.

using TidierData
using StatsBase

@chain df begin
  @slice(sample(1:n(), n())
end

There are two issues here:

• @slice() doesn't allow functions to be run inside of it
• n() may or may not be parsed correctly here.

We need to support functions within @slice(). We may also want to define a local function n() within the @slice() macro that returns nrow() to ensure that n() can be used in functions inside of @slice().

This is not an immediate priority but we should come back to this because it may allow for a simpler implementation of @slice_sample() in the future.

[kdpsingh]

This is fixed in #75.