Improve Issue 10220 fix.

Improves pull #1329.

Issue URL: http://d.puremagic.com/issues/show_bug.cgi?id=10220
Merge pull request #1334 from denis-sh/improve-Issue-10220-fix

Improve Issue 10220 fix.
fix doc comment@
Make unittests pure nothrow for Appender!(raw array)
+
Sharing implementation between ensureAddable and reserve
+2nd Change to @trusted ensureAddable()
+
@trusted constructor
Divide put(U)(U item) for strings and non-strings
Make put to trusted.
-fail to add workaround...
Add CTFEable workaround!
+finish?
++
++y
(doc bug?)