Huge +1 to the general effort. My view is that the lack of file handles and text streaming support in the standard library is a glaring omission that makes using Swift for processing tasks needlessly obnoxious. This is definitely something that we should remedy as soon as we can.
We should provide a byte stream as well as equivalents of all of String's views on top of these streams (performing encoding validation), so that you can read a stream of validated UTF-8 bytes, or Unicode scalar values, or graphemes, or transcoded UTF-16 code units, etc. In the future, when we add normalized views as well, the stream should be able to support that too.
For example, when parsing CSV, you wouldn't want to operate on a stream of grapheme clusters, as grapheme segmentation is irrelevant to CSV and you wouldn't want to have to handle degenerate graphemes. You'd instead want to operate at or below the level of the specification at, i.e. Unicode.Scalar or UTF8.CodeUnit.