[WIP] Initial wasm support

[eminence]

work in progress. CC #161 and @wehlutyk and @Frommi

This takes the work that @wehlutyk did and builds on it by making miniz_oxide_c_api a mandatory requirement if using the wasm32 target. I don't think this can be merged until we get a new release of miniz_oxide, but I was able to test this change (along with the zip crate) to successfully unzip a .zip file in a browser.

I'm not sure if this is the right long-term approach, but I thought I'd open this PR to share with this success with the community.

[wehlutyk]

Very cool! I was waiting for the next miniz_oxide release to start this, which should be coming shortly.

PS: this makes it possible to read compressed pdf files in the browser using lopdf 😄

[alexcrichton]

Thanks! I've pushed up a few commits to tweak things and build wasm on CI as well, all this should need now is a release upstream!

[wehlutyk]

Shouldn't the cfg invocations all be aware that wasm32 can also be emscripten? See eminence#1

[alexcrichton]

Sure yeah, makes sense to me!

[eminence]

Merged eminence#1 !

[alexcrichton]

@Frommi this came up today again in the context of wasm, and was curious, would you be up for publishing Frommi/miniz_oxide#35 to crates.io?

[wehlutyk]

(I'm guessing Frommi/miniz_oxide#37 might want to get in too then)

[oyvindln]

I've merged the first PR, but last one may need more work.

Only @Frommi has publish access on crates.io though.

[alexcrichton]

Oh oops I forgot the rebase would not flag this PR as merged, but this is merged now!