Extend calc example, tutorial with type checker

[nikomatsakis]

The tutorial stops at the parser. We should extend it to include the type checker and tests that show the type checker getting good reuse.

[nikomatsakis]

I'm working on this in my extend-salsa-2022-tutorial branch.

[nikomatsakis]

I'm making various changes to the parser section etc as I go -- for example

• moved statements/expressi5ons to not be interned
• incorporated spans and made some tweaks to them so that they are "relative", thus ensuring good reuse