cache frequently used name strings for DocImpl classes

javac keeps names as interned Name objects. javadoc uses Strings, and creates Strings from Names as needed, sometimes thousands of times per instance. These strings should be cached.

calls/instance of ClassDoc.name: 779
calls/instance of ClassDoc.qualifiedName: 144
calls/instance of PackageDoc.qualifiedName: 10366
calls/instance of PackageDoc.qualifiedName: 1256
calls/instance of PackageDoc.qualifiedName: 1326
calls/instance of PackageDoc.qualifiedName: 149
calls/instance of PackageDoc.qualifiedName: 185
calls/instance of PackageDoc.qualifiedName: 266
calls/instance of PackageDoc.qualifiedName: 309
calls/instance of PackageDoc.qualifiedName: 911