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Callgrind: Subposition compression and weight correction (#423)

This fixes a number of bugs with callgrind import. Dealing with this file format is a big pain because the documentation on https://www.valgrind.org/docs/manual/cl-format.html doesn't contain enough examples to disambiguate some of the behaviour, and because there's a fundamental impedance mismatch between call-trees and call-graphs.

In any case, after this PR, the behavior of callgrind file import is much better.
The file provided in #414 now imports correctly and, as far as I can tell, displays the same weights as what I see in KCacheGrind.

Some of the key changes:
- Implementing subposition compression. This was just a TODO in the code that was never implemented
- Fixing a misinterpretation of how `fe` and `fi` were intended to be used. Previously, I was using it to change the filename of a symbol, meaning that an `fi` or an `fe` line in the middle of a block describing costs for an `fn` would split a node in the call-graph into multiple nodes causing all manners of problems
- Fixing a bug where `cfn` was persisting beyond a single call, also resulting in call graph nodes being split when they shouldn't be

Fixes #414

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