Change serde_json5::from_reader to match serde_json

[erickt]

Serde implements DeserializeOwned on &T, so passing in the reader as &mut R is redundant.

This changes the typaram to match serde_json, so it's a breaking change.

Closes #4.

[coveralls]

Pull Request Test Coverage Report for Build 8758194285

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.285%

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Totals
Change from base Build 8758147368:	0.0%
Covered Lines:	618
Relevant Lines:	677

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💛 - Coveralls