StringBuilder should allow copying directly into other StringBuilders #4606
Description
If you're juggling more than one buffer/builder there isn't an optimal way to move data between them.
Append/Insert/Replace and the constructor should be able to take another StringBuilder to avoid unnecessary allocation of strings/char arrays.
One concrete example is normalizing paths on Windows- it is a potentially two stage process of GetFullPathName and GetLongPathName. Many times the input string will exactly match the output string- keeping everything in buffers allows skipping the final ToString() if it matches exactly.