Remove unused Builder class from aggregation
Remove unused DocumentSourceFilter

It was replaced by DocumentSourceMatch a long time ago.
Simplify aggregation DocumentSource API
Remove unnecessarily defined constructors and destructors from aggregation
Remove unused Value-from-BSONType constructor

Was only used in tests. Using the typed labelers is a better alternative
for new code.
Test hashing Values even if not equal
Add relational operators to Document
Remove deprecated methods from Document
Make ExpressionContext a simple struct

At some point it should probably be renamed since it has nothing to do
with Expressions.
Remove redundant argument number checking

The argument counts are checked by the parser so each Expression doesn't
need to check it's argument counts.