An optional parameter for dump() allows to set the character to use for
indentation (default: space). In case a JSON value is serialized to an
output stream, its fill character is used (and can be set with
std::setfill).
A lot of small changes to avoid memory allocations:
- The locale is only queried once rather than with every number
serialization.
- The indentation string is recycled between different calls.
- The string escape function avoids a copy if no escaping is necessary.
- The string escape and the space function use a complete switch case
instead of cascaded ifs.
Cachegrind measures some 15% performance improvement.
The class is currently just a wrapper for an std::ostream and collects
all functions related to serialization. The next step should be
recycling of variables to avoid repetitive initialization for each
recursive dump call.
