Add parser for configurable workload

[erthalion]

Introduce a new configurable workload parser. The workload definition looks like this:

// Named work block
main (workers = 2, duration = 10) {
  // Anon work block with only one unit.
  // task(name) -- spawn a process with specified name
  // debug(text) -- log with DEBUG level
  // open(path) -- open file by path, create if needed and write something to it
  debug("run task stub");
  task(stub);
  debug("open file /tmp/test");
  open("/tmp/test");
} : exp {
  // If no distribution provided, do the unit only once.
  rate = 10.0;
}

Note that this commit only produces AST without JIT compiling it into anything that looks like an actual code.