Drop all length information from TypeTracer, get all tests working again.

[jpivarski]

@ioanaif, @ianna, and @ctrl-stormy: this is requiring pervasive changes to the Python v2. PRs targeting the C++ will merge well with this, but if you're working on the Python, ask me about it in Slack.

[codecov]

Codecov Report

Merging #1184 (e04c358) into main (9955401) will increase coverage by 1.50%.
The diff coverage is 83.23%.

Impacted Files	Coverage Δ
src/awkward/_v2/_connect/pyarrow.py	83.33% <0.00%> (-0.48%)	⬇️
src/awkward/_v2/_reducers.py	96.48% <ø> (ø)
src/awkward/_v2/forms/recordform.py	66.46% <0.00%> (ø)
src/awkward/_v2/operations/convert/ak_to_numpy.py	100.00% <ø> (ø)
...c/awkward/_v2/operations/structure/ak_unflatten.py	80.00% <ø> (ø)
src/awkward/_v2/types/arraytype.py	80.76% <0.00%> (-0.72%)	⬇️
src/awkward/_v2/contents/unmaskedarray.py	56.27% <51.72%> (+0.82%)	⬆️
src/awkward/_v2/contents/indexedarray.py	59.75% <63.23%> (+0.44%)	⬆️
src/awkward/_v2/_typetracer.py	69.19% <63.58%> (-0.36%)	⬇️
src/awkward/_v2/contents/emptyarray.py	69.54% <74.19%> (+0.26%)	⬆️
... and 31 more