Figure out a plan for architecture support

[luser]

I left some thoughts about this in other issues, but we don't have one on file for this specifically. We're going to want to have support for multiple architectures, so we will need a plan for how to make this straightforward for consumers. My initial idea was to have some sort of Architecture trait that would provide all the necessary details, and then this crate could have built-in types for a few of the most popular architectures (x86, x86-64, arm, aarch64 at least). I was originally thinking we'd have a method like Handler::get_architecture that'd return a type implementing Architecture, but we could also make this an associated type on Handler, which would mean we could do a lot of things statically, like knowing the registers etc, but it might make life complicated for users who legitimately want to support multiple architectures (like my proposed minidump use case).

[tromey]

Multi-architecture is very normal nowadays: an x86-64 program can exec an x86 program. There are also cases where architecture changes between stack frames (some ARM and MIPS things, also SPU which is a bit obscure). So I think it would be better not to bake this into the Handler.