Pre-RFC v6

- Implemented the former Future Possibilities section "Explicit binding" into the main text as "inline implementation environments", mainly in form of grammar extensions.
- Specified that call expressions capture the implementation environment of their function operand, acting as host for implementation-invariant generics there.
- Miscellaneous wording clarifications and fixes. (Turns out you can't call a tuple struct's implicit constructor through a type alias, whoops.)