preliminary ui test examples

[IsaacCloos]

#17

This is an example of a possible file layout for extracting every api misuse from rustc. As elaborated on here it is the design of the rust compiler to express errors in ways that limit trybuild's ability to clump several intra-stack errors together. If I have missed a solution to this please let me know! In the meantime I have separated a handful of read_from api misuses into separate files so that they can all be thrown accordingly and then correctly compared to the relative *.stderr file.

This is in comparison to the zerocopy-derive>tests>ui>[enum | struct | union].rs format where many errors are collected from a single file and can be accounted for concisely.

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I also took the liberty of copying over the #[rustversion] logic from zerocopy-derive. https://github.com/google/zerocopy/blob/e5e9b1fa05933750db526a489f73050032c5eb70/zerocopy-derive/tests/trybuild.rs#L18-L23 Please let me know if you have different plans for zerocopy's tests!

Criticism Welcome 😄

This is a preliminary draft to get a feel for how you'd like these tests to be set up, and get notes on how I've set three tests up already. Thanks!

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