Make Mapper.map_ methods take expr as positional only

[alexfikl]

Came up in inducer/loopy#966 where doing the trick with

map_type_cast: Callable[[Self, TypeCast], Set[str]] = map_constant

only works if the parent class takes the expr argument as positional only. At least I get an error like

loopy/loopy/symbolic.py:1210:5 - error: Method "map_type_cast" overrides class "CombineMapper" in an incompatible manner
Parameter 2 mismatch: base parameter "expr" is keyword parameter, override parameter is position-only (reportIncompatibleMethodOverride)

This updates the base mappers to make the expr positional. Hopefully it doesn't break anything (?).

[inducer]

Fine by me. If it turns something into a crater, it's easy enough to revert.

Thanks!