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.