feat: Add jvm-execution, jvm-ssl and jvm-system modules

[jakubjanecek]

Implementation of JVM-related utilities for std in/out, clock, random
number generation, SSL context initialization and thread pools.

docs: Add section about jvm-* modules

refactor: Add use of jvm-* modules to example

build: Put test dependencies to separate object

[tomasherman]

don't have time to do a proper review right now - for now just couple of points

[jakubjanecek]

I've tried to address all of your comments somehow. Please have another look at it. I removed Clock because they can be created easily so no need for a module (I included them in SystemModule just for simplicity). I did not split the RandomModule and ConsoleModule into separate SBT modules because it seems like too much overhead for nothing.

[augi]

Btw. when I wrote about splitting to more modules, I meant *Module class, not sbt module.