construct character strings with optional new lines

[moodymudskipper]

optionally, for instance c("a\nb") can be constructed as:

c("a
b")

Sometimes that's more readable and we can't use writeLines() or cat() so easily when the string is nested

[moodymudskipper]

However we need to be careful regarding trailing spaces on all lines except last as they're invisible, so we should use the "\U" notation for those.

This will all be handy for construct_diff() too