Added std.range.only - range of a single element.

This adds range type `Only!T` and helper function `only!T(T)`. This is a range that iterates a single value, useful when some interface requires a range, but you only have a single value.
Use RangeError + length optimisation for Only!T
Using version(D_NoBoundsCheck) for Only!T
New docs, examples, added asserts for Only!T
Voldemortification of std.range.only
Changed only.Result to static struct
Undoing formatting error
Merge pull request #1046 from Poita/only

Added std.range.only - range of a single element.
Remove stray // from ddoc.
Complete the fix for issue 8302.

The original fix (e6621da89985ca9688f758c5fffd00438050bfd4) missed the
second variant of std.file.dirEntries.
Merge pull request #1051 from quickfur/std_file_ddoc

Improve std.file.dirEntries ddoc