Fix projectIndex.

Fixes #70.

Need to use a map/reduce until Mongo fixes a projection bug.

Also:
• Improve rendering of FoldLeft and MapReduce source
• Change “expr” to “value”. (Map/reduce forces the “value” label on us, so this keeps us consistent.)
Yep, what looks wrong about it? (That’s why my tests tend to be against `zips` – I run them in the REPL whenever possible.)