Avoid libuv dependency in julia.h

[Keno]

julia.h currently depends on uv.h through support/ios.h. It also declares a number of functions that take uv pointers as parameters. It would be good to decouple this, so we don't include uv.h by default, since it pulls in a number of system headers that may not be compatible with other libraries (e.g. it pulls in some mach headers that can confuse LLVM on OS X).

[tkelman]

ref #8880 #8690 #8754 #9221

[mbauman]

This seems to have been addressed at some point; julia.h no longer includes support/ios.h (nor any header that includes it to my eyes).

[vtjnash]

It is in libsupport.h now