Use PUGI__SCANWHILE for strconv utilities

This makes it easier to optimize strconv. For consistency move all definitions of parser-internal macros to one place.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1007 99668b35-9821-0410-8761-19e4c4f06640
Unroll performance-critical loops 4x

Use a special macro that unrolls the loop body and uses static branch prediction
to improve code generation.

This increases performance across all data sets from benchmark; clang x64 is 10%-40%
faster, clang x86 is 5%-20% faster, msvc is 5%-10% faster.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1008 99668b35-9821-0410-8761-19e4c4f06640