Remove C++11'ism

Remove C++11'ism

While gnu-gcc had `std::map:at const` as non-standard extension
it is n/a for older gcc on OSX.

Surprisingly this const& p() const; performs a tad better as well, likely
due to different exception handling.

Perhaps it is also worth investigating boost::flat_map<> as replacement
for std::map<>, here. Our common case is just a single entry, so using
a std::vector emulated mapping might help.