Bump rust toolchain to 2018-03-24.

[mrowqa]

Newer toolchain contains bugfix required by #14902.

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• ./mach build -d does not report any errors
  It reports some warnings about deprecated items in rust-webvr-api.
• ./mach build-geckolib does not report any errors
• ./mach test-tidy does not report any errors

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This change is 

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @mbrubeck (or someone else) soon.

[mrowqa]

Taking a look at this it seems like NonZero i Zeroable have been removed, so I have left only the manual implementation of it.
CC @nikomatsakis

[mrowqa]

Failed with:

error[E0015]: calls in constants are limited to constant functions, struct and enum constructors
   --> components/msg/constellation_msg.rs:358:71
    |
358 | pub const TEST_PIPELINE_INDEX: PipelineIndex = unsafe { PipelineIndex(NonZero::new_unchecked(5678)) };
    |                                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0015]: calls in constants are limited to constant functions, struct and enum constructors
   --> components/msg/constellation_msg.rs:364:35
    |
364 |     unsafe { BrowsingContextIndex(NonZero::new_unchecked(8765)) };
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

[SimonSapin]

This will likely hit the same WebBluetooth test failures as #20474 (comment) which still need to be investigated.

[SimonSapin]

By the way #20474 already takes care of the nonzero stuff.

[mrowqa]

Oh, I haven't noticed this PR. In this case my change looks redundant.