bitset: Class name and API

[stotko]

Our bitset class is a GPU version of std::bitset which, however, is designed to cover more use cases. In particular, its interface and implementation (run-time fixed-sized) is somewhere between std::bitset (compile-time fixed-sized) and boost::dynamic_bitset (run-time dynamic-sized). This may lead to confusion if users expect the exact same API as std::bitset.

There are several ways to address this issue:

• Rename it to dynamic_bitset and extend its API to match (as close as possible) boost.
• Rename it to vector<bool> and change/extend its API to match (as close as possible) vector.
• Keep the name and extend its API towards boost's version, i.e. considering the non-dynamic-sized functions.

At the moment, the last option seems to be a good compromise. However, it does not fully solve the problem regarding potential user confusion. Since any of the options will break the API, this change is considered for stdgpu 2.0.0