Document std.range.transposed (issue 4330).

Add unittested example as well.
Fix issue 8764

Transposed can only work with ranges of ranges if the subranges are
assignable. Add signature constraints.

Fix forward range bugs (should use .save instead of just copying the
range object).

Add unittests.
Move docs from struct Transposed to transpose().
Don't use fprintf to output D strings since they might contain embedded nulls.

Instead, use fwrite to output the string in its entirety. It's probably
more efficient this way anyway, since we bypass format string parsing in
fprintf.
Update unittest to check for correct writing of embedded nulls.
some love for std.string

many functions of std.string had no attributes like pure, safe etc.
additionally, many const arguments where not marked as const.
tabs -> whitespace
removed tail whitespace
Doc Examples to unittest docs

more love for std.string

const to in

squeeze
Merge pull request #1138 from quickfur/transposed

Fix issue 4330 and issue 8764
Merge pull request #2323 from burner/std_string_attributes

some love for std.string
Merge pull request #2334 from quickfur/issue8730

std.stdio.writeln should write strings with embedded nulls correctly