Correctly handle last lines with spaces

[Lucus16]

Fixes #78

[Lucus16]

Hmm, this introduces some unnecessary simple strings:

-      description = ''
-        A macro to generate structures which behave like bitflags.
-      '';
+      description =
+        "A macro to generate structures which behave like bitflags.\n";

[Lucus16]

Fixed now.

[Lucus16]

@kirelagin Could you get this reviewed and merged?

[Heimdell]

The whitespace detection lacks tabs, and some derivations use it. This might produce incorrect output.

[Heimdell]

Why not isSpaces, like in the rest of the code?

[Lucus16]

This function is specifically to catch an edge case where strings are better represented as a simple "" string, rather than an indented '''' string. It is not about matching the behavior of Nix. I added this because nixfmt used to turn things like "\n\n" and "\n " into indented strings.

[Heimdell]

What if there are tabs? NixOS/nix#2911

I suggest replacing it with Text.all Data.Char.isSpace (it even is already imported!).

Suggested change

	isSpaces = Text.all (==' ')
	isSpaces = Text.all isSpace

[Lucus16]

Nix handles lines consisting entirely of spaces different from lines that contain other kinds of whitespace. This function is intended to match that behavior but it could probably be better documented. I got stuck on finding good names for lines without any characters vs lines consisting of only but at least one space vs lines that could be either.