SERVER-10210 Use Snappy compression when Sorter writes to disk
SERVER-9443 TopKSorter optimization when spilling to disk
Replace ExpressionNary::getFactory() with isAssociativeAndCommutative()

Intermediate step in agg Expression refactor. I think most of this code
will be gone before the refactor is finished, but I needed to make this
a step along the path.
Unify agg Expression parsing to eliminate special cases

Also fixes SERVER-9289: allowing objects as unwrapped single arguments
Replace static, sorted list of expressions with localized registration
Use bulk-insert in $out

About 4x faster at simple copy-collection benchmark.
Follow-up to SERVER-3253
SERVER-10530 Break first batch in aggregation cursor when encountering a large document
remove useless file
Update comments in ExpressionNary::optimize()
Use BSONElement::fieldNameStringData() in aggregation