Put default value within aggregate

[vmx]

Prior to this change, default values for aggregates were specified
as:

max(.foo) default=5

The new syntax is putting the default value within the aggregate
function:

max(.foo default=5)

This makes more sense as the default value is not a single value
for the max() function, but it is applied to every Keypath
within the max() function.

It is also more symmetrical to the default values for returns.
There it's also , e.g.:

return .foo default=5

This is part of #51.