fix: Columns and Values should recognize Slice values too

[max-stytch]

Continues work done in #67 with the same rationale - this worked in 1.3 and no longer works in 2.0. This fix allows us to continue to use scan to pull the column names and values out for slice and array elements.

A more concerete example:

We use arrays for lots of filter operations - for example if I wanted to find all Widgets owned by a group of users, I might define

type WidgetFilter struct {
    OwnerIDs []string `db:"owner_id"`
}

and then

WidgetFilter{OwnerIDs: []string{"123", "456"}

would be evaluated as

select * from widgets where owner_id in (?, ?), ["123", "456"]

We're hoping to release some of this stuff that builds on scan soon!

[coveralls]

coverage: 99.627%. remained the same when pulling b312ee8 on max-stytch:max/fix-scan-slices into 9c197bf on blockloop:master.

[blockloop]

LGTM