Use cElementTree to reduce memory consumption

[tautschnig]

Description of changes:

Parsing CBMC's results in a memory consumption that is 100 times the size of the input file. Switching to cElementTree appears to halve the memory footprint, which can make the difference as to whether this fits into host memory. We could reduce the memory footprint a lot by iterating (and cleaning up) over the input XML nodes, but this requires a much larger re-architecting of the source.

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