Auto merge of #12918 - hsinewu:12902-typedefs, r=Manishearth

12902 typedefs

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Using type alias instead of newtype.
Removing duplicated implementation, it's already inherited.
No more type constructors and foo.0
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12902 (github issue number if applicable).

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- [X] These changes do not require tests because it's refactoring.

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