Breaking Changes

[CraftSpider]

There was discussion in #56/#57 about breaking changes, upping MSRV, etc. I wanted to collect thoughts/ideas/etc somewhere, and a list of things desired for a next update.
@waych / @pkgw feel free to comment on anything to add/remove from this list.

• Bump MSRV
  • Hard to find really good numbers, but https://lib.rs/stats#rustc implies that of the top 4000 crates, only a handful compile on rustc older than 1.31, so if we want to be 'maximally compatible', that might be a good number
  • Alternatively, 1.40 gets us #[non_exhaustive], which would help reduce breakage by allowing public structs and enums that can be later extended.
  • 1.56 is the next major bump in how many crates support it, so is probably the most logical choice after the above
• Delete deprecated things
• Fix NotMSVC error codes / spaghetti target code
• Improve type extensibility, make some stuff private
• General cleanup - General code cleanups #57 is a start on this
• Add support for determining library versions - Add code to expose the selected version of a requested package #56, on the list because it requires breaking Library

[pkgw]

I'd recommend aiming for at least 1.40 for non_exhaustive — it would be nice for a breaking release to be "lightly breaking", where hopefully a lot of consumers will be able to update without changing any code. To be more future-proof with Library without non_exhaustive would require a more invasive change, AFAICT.

For the spaghetti target code: the code in tectonic_cfg_support should hopefully be a nice reference for properly handling the CARGO_CFG_TARGET_* variables; my impression has been that a failure to properly respect those has been a source of a lot of the spaghetti-ness there.