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().
Fixed handling of a struct with no "length" field defined but with opDispatch of inappropriate type.
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
prevent ambiguity when calling take with a Take!R

If Take!R satisfied ```isInputRange!(Unqual!R) && !isInfinite!(Unqual!R) && hasSlicing!(Unqual!R)``` then you would get an ambiguity error calling take!(Take!R)
fix unittest
actually fix the unittest...
Merge pull request #2340 from John-Colvin/patch-9

prevent ambiguity when calling take with a Take!R
Merge pull request #2062 from yglukhov/patch-1

Fix Variant to support structs with "length" field of inappropriate type
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