GenericReader: simplify local stream copy optimization

rapidjson.h:
 * StreamLocalCopy: add default argument to copy optimization selector
   based on StreamTraits of Stream parameter
 * drop operator->, operator*
 * make Stream (reference) member public
 * drop empty destructor

reader.h:
 * add local references, initialized from "copy"
   (reverts algorithmic bodies back to plain 's.xx()')
StreamLocalCopy: move to reader.h

Eventually move to internal header `internal/streamcopy.h`?

Based on 088e8173.
Merge pull request #42 from pah/cleanup/streamcopy

GenericReader: simplify local stream copy optimization