Add public constructor for public structs

[TedBrookings]

Hi, I'm using rust_htslib to parse XA tags and wanted to use IterAlignedBlocks to find where the alt alignments pointed. Unfortunately the fields are private so it's not possible to construct that struct outside it's module. From what I can tell the easiest way to fix this is to add a public constructor. I'm thinking that this might be a good practice for many of the structs that are deeper in the project, because it saves people from having to reinvent the wheel.