* Fix String::replace()
Fixes#5883 and supercedes #5890
The replace() function was using len() while in the middle of buffer
operations. In SSO mode len() is not stored separately and is a call to
strlen(), which may not be legal if you're in the middle of overwriting
the SSO buffer, as was the case in ::replace when the replacement string
was longer than the find string. This caused potential garbage at the
end of the string when accessed. Instead, just cache the length in a
local while doing the operation.
Add in test cases from #5890 as well as some new ones that fail on the
unmodified core.
* Fix stack smashing error on 64b
When pointers are 8 bytes long, the size of a String is larger than 16
chars. Increase the allocated array we're using in the test to avoid a
"stack smashing" error.
* Manually call destructor in test
Just for clarity, manually call the destructor for the Strings() that
are "placement new'd" in the String tests. It is a no-op for the
existing test, since thanks to SSO there are no memory allocations, but
will help in case someone adds tests later which include longer strings.
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