* Rust-crypto: fix `bootstrapCrossSigning` on second call
Currently, `bootstrapCrossSigning` raises an exception if it is called a second
time before secret storage is set up. It is easily fixed by checking that 4S is
set up before trying to export to 4S.
Also a few logging fixes while we're in the area.
* Factor out an `AccountDataAccumulator`
* Another test for bootstrapCrossSigning
