fix potential class/struct odr violations, and fix 32-bit shift

[iboB]

Two commits in this PR:

• Fix potential ODR violations from the same types declared as class or struct. While it's true that the Itanium ABI, does not distinguish between the two, other ABI-s might, and MSVC's does. In rare cases this may lead to crazy bugs.
• Fix a 32-bit shift: 1 << M is a 32-bit shift. So, even though M and buckets will be 64-bit (size_t) on a x64 system, when M is 32 or more, UB will ensue.

[jll63]

Sorry I just noticed this. I was busy with transforming YOMM2 into Boost.OpenMethod. I am about to announce (on reddit) that I am retiring YOMM2 - but my work on open methods continue of course!

Thanks for the PR though. class vs struct is a Microsoft blunder. I did fall prey to strange bugs on MSVC. I'll check if the shift problem exists in OpenMethod. I am merging your PR and you will have the honor of submitting the last one.

Please consider moving on to Boost.OpenMethod!