Revert "do not use a nested class for Backtrace::Frame"
Revert "do not use a nested class for Backtrace::Frame"

This reverts commit 138213ad4c11320b1213716afcfdc05a90141cbe.
On my system it causes S02-literals/autoref.t and
S02-literals/pairs.t to each run for over two minutes
(consuming a fair bit of memory), and halts the rest of
the spectest process.

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