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.

[Molter73]

Oh lord, self referential structs/enums, this is going to be fun.

Mandatory "if you get into trouble you should read https://rust-unofficial.github.io/too-many-lists/" PSA.

[erthalion]

Hm? The page you've posted doesn't mention self referential structures at all, am I missing something?
And even if it's somehow bad in Rust, what is your suggestion? Parsing AST without self references is like trying to hammer a nail without an actual hammer -- you probably can do this, but it will likely be very awkward.