Add default.nix file, for building with Nix
The option --permissive now quiets balance assertions
Add the concept of "deferred postings"

This is pretty much exclusively for allowing one to use balance
assertions with replicated transactions across multiple files.
Update tools/build.sh
Bump the date in the version number
Apply any outstanding deferred postings at the end of parsing
Correct one of the baseline tests
Add a safety check when using deferred postings