Improved INSERT/UPDATE syntax

E.g.,

    users.insert(name <- "Alice", admin <- true)

I know, I know, this goes beyond operator overloading into custom
operator territory, but hear me out! You don't NEED to use the
operator. I think it works particularly well here, but if you protest:

    users.insert(set(name, "Alice"), set(admin, true))

In any case, this is an improvement on the existing state of things. A
block-using builder just to set a bunch of values is a cumbersome
affair. We needn't use blocks where we can avoid it.

Suggested by Thomas Denney <http://programmingthomas.com>.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Add documentation to Query methods

They were lost in the Expression refactor shuffle.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Comments can be more descriptive with even one letter

Signed-off-by: Stephen Celis <stephen@stephencelis.com>