Allow do to execute procedures passed to it

[zephraph]

I ran into a case where I'd like to execute a nested procedure. I'd already added similar functionality inside of match and ultimately most of the operations should accept procedures as well as functions.

Here's what this looks like:

// First we'll define a procedure to be passed into `do`
const nestedProcedure = procedure("nested procedure").do(something);

// Then we'll call the procedure w/ do
await procedure('parent procedure').do(nestedProcedure).exec({});