Inverse of `.MapTo<'T>`

首先查看现有的 MapTo<'T'> 用法以及 issue 中展示的 Create/SubmitUpdates 流程。确定将记录转换为实体所需的 API 和映射行为，包括示例应如何在不进行逐列赋值的情况下工作；当行为及其支持的情况已定义并验证后，即视为完成。

Is your feature request related to a problem? Please describe.
This library provides the .MapTo<'T> to map an entity to a record type (which is awesome), but it is not as simple to map this record type back to an entity. Mapping back can be done manually, but it can get a little cumbersome when the entity has a lot of columns.

Describe the solution you'd like
It would be cool to have some sort of inverse of the .MapTo<'T> method that could take a record and return an entity.

Describe alternatives you've considered
As mentioned above, it can currently be done manually. This works fine for small entities, but scales with the number of columns in the entity.

Basic Example
If we have a table MyTable with column Id that is mapped by the record MyTable_Row, you can easily grab a row based on the column Id using

let getById (id: int): MyTable_Row option =
    let foundRow = query {
        for row in ctx.Dbo.MyTable do
        where (row.Id = id)
        select (Some row)
        exactlyOneOrDefault
    }
    match foundRow with
    | Some foundRow -> Some( foundRow.MapTo<MyTable_Row>() )
    | None          -> None

This method is independent of the number of columns in MyTable (other than in the definition of MyTable_Row). But, if we want to add a row to MyTable from a MyTable_Row, we need to manually specify that each column gets populated:

let insertRow (myRow: MyTable_Row) =
    let newRow = ctx.Dbo.MyTable.Create()
    newRow.Id      <- myRow.Id
    newRow.Column1 <- myRow.Column1
    newRow.Column2 <- myRow.Column2
    newRow.Column3 <- myRow.Column3
    newRow.Column4 <- myRow.Column4
    ...
    newRow.ColumnN <- myRow.ColumnN
    ctx.SubmitUpdates()

With this, not only does the definition of the record type need to scale with the number of columns in the entity, but all functions performing inserts must too.