This change makes document building --- and specially incremental
document building --- more scalable. The global duplicate-definition
check is handled by a database query, for example.
original commit: 9888fac99e32131a6e848f30c40024abb893a9b9
The `xref' produced by `setup/xref' uses the database to delay
loading "out.sxref"s, which cuts 64-bit DrRacket's initial
footprint by around 50MB (i.e., about 20%).
original commit: 8c1b5db81553b54b35e753441efd0465426707a3
