Improve Python generator EDSL

[sampsyo]

The first version of the weird little Python EDSL for generating Calyx programs in #1229 leaves a lot of gaps! Here are some ideas for what to do to it next; please feel free to add.

• Convenience for more control operators (par, while, if). (It is already allowed to construct the AST explicitly.) par should use Python `sets.
• More coverage of stdlib components, and a way to construct user-defined (non-stdlib) components.
• Continuous assignments. Probably do something like with comp.continuous:, or just have this be the default when you're not in a with.
• @cgyurgyik's idea to add simple correctness checks for well-known stdlib cells: for example, throw an error when assigning to an unknown port. (There should be an easy way to declare the set of known ports for a given component.)
• Explore @rachitnigam's idea re. swapping out the "backend" to generate in-memory IR instead of serializing the AST to text.
• Documentation page.