Improve std lib

[BinderDavid]

Add new types Sigma, Pi and Vec, and make type arguments to ap implicit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

see 6 files with indirect coverage changes

[timsueberkrueb]

One note: we decided to use ASCII for type names and {con, de}structors and define unicode notation separately. We can already do this using transparent let bindings even though we lack first-class support for user-defined syntax.