slurp files and http URLs, refactor rust_core structure

[erkkikeranen]

(slurp "example.txt") file contents

(slurp "doesn't exists.txt") #Condition["No such file or directory (os error 2)"]

(def s (slurp "example.txt")) ; works

(def s (slurp "http://www.example.com"))

Also refactored rust_core to separate files

[Tko1]

Looks good. I don't know if slurp needs to be in another file -- rust_core, the file, represents that entire namespace of functions, just as clojure/core.clj does. Often it really just depends, I could maybe see us splitting it into its own file -- if we split all the rust_core functions into their own files. But then, they don't each need to be their own module, each their own thing to be imported to be used. Splitting up the files would be more about being able to see more of the structure of rust_core by looking at the file system, to be able to see each function and macro at a glance there (which helps if your editor doesn't easily display the list of functions in the file you're looking at) -- but overall I don't think that's a problem. You might say its also about being able to look at each function in isolation, but because they have one consistent, repeated structure, rust_core can just be seen as a list of fns and macros, and there's a clear delineation between the function definitions -- you can look at each one in isolation pretty easily there.

Anyways, so for now its preferable to have slurp in rust_core.rs

[Tko1]

As mentioned on discord, the two ways of breaking up the files for rust_core have tradeoffs, and even though I have reasons for preferring the other way, it shouldn't be significant enough to matter -- so to merge this PR request

1. Add this to rust_core.rs instead
   or
2. Split up rust_core.rs into files so that it matches this