handle more grafting in libraries

[jan-ferdinand]

Currently, converting a rust type to a built-in includes manual checks against certain hardcoded, “well-known” built-ins.¹ The number of these built-ins is predicted to rise,² and special casing all of them seems both unclean and error-prone.

Handle grafting in the corresponding Library implementations where necessary. See also #51.

Footnotes

1. namely Vec and Box ↩

2. add a Result type #49, add an Option type #53 ↩