wasm: implement imports

[sbinet]

Updates #40.

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This change is 

[vibhavp]

In the future, we could allow wasm.Module to have Go-native functions and variables.

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Review status: 0 of 5 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[sbinet]

yep, that would be great :)
(so the last step of my advent-2017 blog post could be done in pure Go instead of a web server executing that wasm module or reaching for nodejs to do the same...)