Single case union support

[lucasteles]

Proposed Changes

Single case union types (aka Simple types)

It's common to use single case discriminated unions to meaningfully represent data values.
EF does not know anything about this kind of type. This PR enables migration and mapping of single case discriminated unions

Example:

type PostitiveInteger = PositiveInteger of int

[<CLIMutable>]
type Blog = {
    [<Key>]
    Id : Guid
    Title : string
    Votes: PositiveInteger
}

type MyContext () =
    inherit DbContext()

    [<DefaultValue>]
    val mutable private _blogs : DbSet<Blog>
    member this.Blogs with get() = this._blogs and set v = this._blogs <- v

    override _.OnModelCreating builder =
        builder.RegisterSingleUnionCases()


let sample ctx = 
               query {
                   for blog in ctx.Blogs do
                   where (blog.Votes = PositiveInteger 10)
                   select blog
                   headOrDefault
               }

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Build and tests pass locally
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

[simon-reynolds]

Ties into #66

[simon-reynolds]

Hi @lucasteles
I like this change a lot! Can you resolve the conflict please and I'll approve and merge

[lucasteles]

Resolved!