C++ refactoring: Make highlevel __repr__ safe for typetracers.

[jpivarski]

@douglasdavis, here's what it looks like with only the type section (no values section).

>>> import awkward._v2 as ak

>>> x = ak.Array([[1, 2, 3], [], [4]])
>>> ak.Array(x.layout.typetracer)
<Array-typetracer type='?? * var * int64'>

>>> x = ak.Record({"x": [1, 2, 3], "y": 1.1})
>>> ak.Record(ak.record.Record(x.layout.array.typetracer, 0))
<Record-typetracer type='{x: var * int64, y: float64}'>

[codecov]

Codecov Report

Merging #1218 (c0bff9d) into main (0a0e9be) will decrease coverage by 0.07%.
The diff coverage is 67.12%.

Impacted Files	Coverage Δ
src/awkward/_v2/_typetracer.py	68.94% <33.33%> (-0.25%)	⬇️
src/awkward/_v2/highlevel.py	64.43% <33.33%> (-0.56%)	⬇️
src/awkward/_v2/_slicing.py	83.79% <75.47%> (-2.48%)	⬇️
src/awkward/_v2/contents/regulararray.py	82.66% <80.00%> (+0.07%)	⬆️