Bugfix release v1.3.6

• Refactoring for #73, enhanced performance in path handling. the changes lead to much fewer path/string creations or copies, speeding up large directory iteration or operations on many path instances.
• Bugfix for #72, the TestAllocator in filesystem_test.cpp was completed to fullfill the requirements to build on CentOS 7 with devtoolset-9. CentOS 7 and CentOS 8 are now part of the CI builds.
• Bugfix for #70, root names are now case insensitive on Windows. This fix also adds the new behaviour switch LWG_2936_BEHAVIOUR that allows to enable post C++17 fs::path::compare behaviour, where the comparison is as if it was an element wise path comparison as described in LWG 2936 and C++20 [fs.path.compare]. It is default off in v1.3.6 and will be default starting from v1.4.0 as it changes ordering.