WIP: readlink function

[ghost]

Necessary for #10506 and simply wraps libuv. This is my first time working against libuv and working with ccall, any feedback is very much appreciated. Also, should we export readlink? For now I only exported it from Base.FS.

[ghost]

Failing due to white space errors in a commit with [ci skip]. Preparing a separate PR to fix this.

[ghost]

CI run now pending on #10572.

[tkelman]

I restarted the travis build for you. Though I think it might be better to add a wrapper function in C in jl_uv.c, so the Julia-side ccall version is simpler. That's the approach I took in #7590 for example.

[tkelman]

And as far as exporting goes, considering the number of other filesystem-related functions we currently export, I think exporting readlink would be good for now. Un-exporting portions of FS might be good to do eventually, but probably best to do a bunch of them at the same time, and export this for now.

[Keno]

Either this or a separate wrapper in jl_uv.c is fine to me (a separate wrapper is probably slightly better since you can put the fs struct on the stack, but you're doing a system call anyway, so it doesn't matter much). Also needs docs though.

[ghost]

Should this be closed in favour of #10714? I am now back from a conference and would be fine with finishing this, but it seems that there are two independent efforts at this point.

[tkelman]

Sure, though your input there would of course be welcome. It might be useful to keep your all-Julia-side implementation of readlink around for reference, since I'm not sure exactly how much of the function we will actually want to do in C vs Julia for the final version.