Optimize unrolled scanning for MSVC

While gcc and clang can eliminate dependency on s in the inner loop of
PUGI__SCANWHILE_UNROLL, MSVC emits a series of register increments.
Rewriting the code to explicitly remove the dependency keeps similar
codegen on gcc/clang but improves codegen on MSVC for a 10% performance
boost.

Also use unrolled scanning in text_output_escaped (2% faster).

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1048 99668b35-9821-0410-8761-19e4c4f06640