Miscellaneous Improvements

[Cgettys]

I came across wrapped_mono through your article. It looks interesting, and I noticed some small fixes here and there I could easily contribute, so here I am, contributing them :).

Happy to split out any commits you want to handle individually, and feel free to ignore these. Tests still pass, except the proc macro doctests, which I never got to work on my system and did not make any related changes to.

The commit messages give a good overview, but to summarize:

• Expand .gitignore to not include Jetbrains IDE files that don't belong being checked in (to prevent me from accidentally checking in said files)
• Some formatting / wording tweaks in the readme
• Some typos in traits and feature names (InteropRecive -> InteropReceive, referneced_objects -> referenced_objects
• Fix warnings about unexpected feature old_gc_unsafe by using feature = "old_gc_unsafe" - recent change to make this warn, there was a blog post somewhere explaining.. https://doc.rust-lang.org/rustc/check-cfg.html is relevant here as well
• Propagate write! failures via ? instead of silently producing the wrong generated code (this is the only functional change in the PR that I'm aware of)
• Remove a few unnecessary mut keywords
• Some minor assert!(a == b) to assert_eq!(a, b) and likewise for != to assert_ne

You might want to consider making src/lib.rs doctest embedded from readme.md, like rust-lang/rfcs#1990