Suggested improvements for pipes extension

Not all feedback on #635 was implemented for the current version of the extension, so this issue is to track the outstanding suggestions:

1. Additional class interface, such as:
```c++
public:
using value_type = dataT;
std::size_t constexpr min_capacity = MinCapacity;
```
and probably with other container-like types: reference...
* https://github.com/intel/llvm/pull/635#discussion_r324952087

2. "have instead of std::size_t a numeric type as a template parameter" for the pipe class
* https://github.com/intel/llvm/pull/635#discussion_r324953479

3. More C++-like interface for host pipe map/unmap/read/write
* https://github.com/intel/llvm/pull/635#discussion_r324955956
* https://github.com/intel/llvm/pull/635#discussion_r324956992