Concerns that an inconsistency could arise between the actual contents
of the capsule in revlog.c and the Rust consumer have been raised after
the switch to the array of data and function pointers in f384d68d8ea8.
It has been suggested that the version from parsers.c could be use for
this. In this change, we introduce instead a separate ABI version number,
which should have the following advantages:
- no need to change the consuming Rust code for changes that have nothing to do with the contents of the capsule
- the version number in parsers.c is not explicitely flagged as ABI. It's not obvious to me whether an ABI change that would be invisible to Python would warrant an increment
The drawback is that developers now have to consider two version numbers.
We expect the added cost of the check to be negligible because it occurs
at instantiation of CIndex only, which in turn is tied to instantiation
of Python objects such as LazyAncestors and MixedIndex. Frequent calls
to Cindex::new should also probably hit the CPU branch predictor.