Introducing RazorTextWriter

RazorTextWriter represents the result of rendering a page as a sequence of
strings rather than a concatenated string. This avoids building up large
strings in memory.
Make HTML helper `null` handling consistent
- #874 lines 3, 4, and 6
- correct `Value()` to treat a `null` expression name the same as `string.Empty`
- add missing `[NotNull]` attributes in `EditorExtensions` and for `GenerateIdFromName()`
- consistently pass `null` for default expression names to the helpers
 - for example, from extension methods
- add test cases using `null` for expression name

nits:
- correct summary XML comment for `HtmlHelper` class
- use named parameters and prefer interface (not extension) methods in changed calls
- use `string.Empty` instead of `""` in a few tests