Change TagHelperOutput Content, PreContent and PostContent to all be TextWriters to avoid large string allocations.

[NTaylorMullen]

This issue is the more concrete version of #295.

What should be done to TagHelperOutput:

1. Change TagHelperOutput.PreContent to be TagHelperOutput.PreContentWriter.
2. Change TagHelperOutput.Content to be TagHelperOutput.ContentWriter.
3. Change TagHelperOutput.PostContent to be TagHelperOutput.PostContentWriter.

All new property names here should be of type https://github.com/aspnet/Mvc/blob/dev/src/Microsoft.AspNet.Mvc.Core/Rendering/StringCollectionTextWriter.cs but exposed as ITextWriter. This means that type will need to be transitioned into the Microsoft.AspNet.Razor.Runtime repo.

Extra additions to make this change more consumable:

1. ITextWriter should have an extension method to write on ITextWriter into another ITextWriter.
2. TagHelperContext's GetChildContentAsync should have an override to provide your own ITextWriter to write to.

The impact of this change will end up affecting all existing TagHelper's and will greatly affect how TagHelpers used (NOTE: Lots of impact in MVC to make this plausible).

[NTaylorMullen]

@sornaks: @yishaigalatzer, @DamianEdwards, @dougbu, and I felt like this is a pretty large feature that should make it in sooner rather than later (since it has huge impact). We can sit down together tomorrow or when you're ready to pair program the beginning parts of this to give you enough context on what needs to be done (lotssss of pieces to this 😄).

[dougbu]

All new property names here should be of type https://github.com/aspnet/Mvc/blob/dev/src/Microsoft.AspNet.Mvc.Core/Rendering/StringCollectionTextWriter.cs.

Do you mean "of type ITextWriter which we (in MVC) back using StringCollectionTextWriter"? If yes, StringCollectionTextWriter might not need to move.

[NTaylorMullen]

Yes, muchas gracias 😄