enable natural setters on data tables

[smoothdeveloper]

DataTable types coming from SqlCommandProvider don't have natural setters for the column accessor.

The work around is kind of unatural / unwieldy:

type MyCommand = SqlCommandProvider< myConnection, myQuery, ResultType = ResultType.DataTable >

let table = MyCommand.Table()
let row = table.Rows.NewRow()
table.Columns.column1.SetValue(row, "my value")

proposed:

type MyCommand = SqlCommandProvider< myConnection, myQuery, ResultType = ResultType.DataTable >

let table = MyCommand.Table()
let row = table.Rows.NewRow()
row.column1 <- "my value"

related: #215, #221

If anyone is opposing this change, please let me know.