Simplify prompts.template and make it composable

[rlouf]

We want a better interface for prompts that does not use too much magic. Essentially what we want is a simpler, modular interface to jinja2 plus some magic to avoid prompting mistakes

import prompts


@prompts.template
def new_template(username: str):
    return "{{ prompt }}"

What the decorator does:

1. Gather function signature
2. Execute the function
3. Gather argument values and bind to variable names. Apply defaults.
4. Render the Jinja2 template using the variables detected by finding undeclared variables, if any.

This allows composition (#7)