api/queryinternal.cc,matcher/exactphrasepostlist.cc: We never try to
build an ExactPhrasePostList over 0 or 1 terms, so simply assert we
don't, rather than having an explicit check for it in the code.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18285 6b2f1b92-a4f3-0310-943f-de3cbe41748b
api/queryinternal.cc,matcher/: Split NearPostList out into separate
source file and header.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18286 6b2f1b92-a4f3-0310-943f-de3cbe41748b