clean up JuliaIsExist.

Specifically:

*Rename JuliaIsExist to JuliaExists;
*Document internal function workings;
*Standardise R style with the best practises recommended in "Advanced R Programming" and generally render more readable;
*Use a single Sys.getenv call and a series of similarly vectorised operations, rather than a long control flow;
*Rename internal variables to be more readable (less "ret", more "julia_found");
*Rely on regular expressions for the detection of trailing slashes or the lack thereof - a single R string is actually a character vector of length one. It can't be subsetted with [1],[2] etc in the same way that a similar C or Python string could.
*Modify subsequent calls to JuliaExists to reference the new name

If I've missed anything, lemme know.