enum class chars_format

[dalle]

Changed chars_format to an enum class.
Added operators to comply to BitmaskType.

Fixes #278

[lemire]

@dalle What would you think about specifying uint64_t as the underlying type and casting to uint64_t? As the code currently stand, it would make no difference, but I tend to prefer fixed-size integers (for easier portability).

[dalle]

I'm all for it. uint32 or uint64?

[lemire]

I'm all for it. uint32 or uint64?

I usually default on uint64_t in such cases. Going for 32 bits saves nothing in this case.

[lemire]

Merging.