Switch to abseil symbolization / stack tracing routines

[asl]

This removes dependency on exeinfo and instead delegates platform-specific code to Abseil. The libbacktrace code is left intact as it provides slightly better information (file / line numbers when debug info is available), however, demangling there is switched to abseil::Symbolize is it does nice template type names filtering.

[asl]

Example of Abseil's typename filtering during demangling:

allocated  80MB in 24992 calls from:
 ??: absl::lts_20240116::container_internal::raw_hash_set<>::insert<>() [100977894]
 ??: absl::lts_20240116::container_internal::raw_hash_set<>::find_or_prepare_insert<>() [100986820]
 ??: absl::lts_20240116::container_internal::raw_hash_set<>::prepare_insert() [100986910]
 ??: absl::lts_20240116::container_internal::raw_hash_set<>::resize() [100984fbc]

[ChrisDodd]

Looks good overall -- I wasn't really happy with adding the dependence on abseil in the first place, but given we already have it.