feat(ir): support showing variable names used to create an expression in repr()

[kszucs]

For the following code:

import ibis

alltypes = ibis.table(...)

filtered = alltypes[alltypes.f > 0]
ordered = filtered.order_by("f")
projected = ordered[["a", "b", "f"]]

expr = projected.a + projected.b

ibis.options.repr.show_variables = True
repr(expr)

instead of showing

r0 := UnboundTable: alltypes
  a int8
  b int16
  c int32
  d int64
  e float32
  f float64
  g string
  h boolean
  i timestamp
  j date
  k time

r1 := Filter[r0]
  r0.f > 0

r2 := Sort[r1]
  asc r1.f

r3 := Project[r2]
  a: r2.a
  b: r2.b
  f: r2.f

Add(a, b): r3.a + r3.b

now we can render the expression representation as

alltypes := UnboundTable: alltypes
   a int8
   b int16
   c int32
   d int64
   e float32
   f float64
   g string
   h boolean
   i timestamp
   j date
   k time

 filtered := Filter[alltypes]
   alltypes.f > 0

 ordered := Sort[filtered]
   asc filtered.f

 projected := Project[ordered]
   a: ordered.a
   b: ordered.b
   f: ordered.f

 Add(a, b): projected.a + projected.b

This removes one level of indirection when we inspect an expression. Also not just tables but value nodes assigned to local variables are shown.

Note that it has its limitation based on where repr() is called, but ibis.expr.format.pretty now accepts a scope argument where the variable names can be passed.

[kszucs]

Requires a follow-up ticket to document this.

[cpcloud]

Seems fine, but doesn't really work with IPython at the moment.

I'm a bit wary of testing this until we work out more of the kinks given how niche the feature is.