This change adds an encoded version of hygiene related datastructures to
the metadata of a crate. It is then decoded on-demand when spans and AST
items from a foreign crate are accessed. Some changes might still be
necessary to implement the correct behaviour, however.

Sadly, this isn't the reason the tests fail.

However, now that it behaves as expected, the memory usage is
unacceptably high. A solution would be more lazyness during encoding or
a pruning scheme.