Shrink the size of all `Error` types

[alexcrichton]

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This commit improves the in-memory size of Error from 7 pointers to 1 pointer. Errors are in general relatively rare in applications and having a huge error type ends up generating lots of instructions for moves and such, so this PR optimizes for size and passing around errors rather than creating errors.

[Yamakaky]

Nice! You just have to update the tests which use pattern matching on the kind. It's also a breaking change so could you update the CHANGELOG?

[alexcrichton]

Updated

[Yamakaky]

Could you rebase please?

[alexcrichton]

Sure thing, done now

[Lymia]

When might this pull request be land/be merged?