Add support for --run

[jonascarpay]

Initial attempt at #56.

• Argument handling is provisional; it just checks if the argument list is of the form --run <str>, and if so, tries to run the test.
• Running the test means parsing into a module name and function name, and then calling nix-instantiate --eval -E "(import ./<moduleName>).<functionName>".
  • @considerate In your example, you pass null. Why is that?
• I split the Main module into Main.Build and Main.Run.

[considerate]

In the example I pass null to run the Effect. I think inspecting whether it’s a function and calling the function with null would be a strategy that could work.

[considerate]

For reference, spago always runs the main function and calls it without any argument (equivalent to calling with null in purenix) for the JS backend.