Auto merge of #422 - marmistrz:wrappers2, r=nox
Auto merge of #422 - marmistrz:wrappers2, r=nox

Create a second set of borrowing wrappers

As agreed here:
https://mozilla.logbot.info/servo/20180326#c14521098-c14521118
https://mozilla.logbot.info/servo/20180405#c14564933

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servo/rust-mozjs,servo/rust-mozjs,servo/rust-mozjs,servo/rust-mozjs