Docstring support

[OAGr]

Many programming languages have good support for docstrings. It could be great to have them in Squiggle for explaining reasoning.

// For someone of my age range, I'm expected to live another 30 to 80 years.
timeToDeath = years(30 to 80)

Docs.doc(timeToDeath) // -> "For someone of my age range, I'm expected to live another 30 to 80 years."

Later we could make this more fancy.