Revert "Implement core prim toString methods"

This reverts commit aeb9d16a5e184cb15858fc879be920f47d180ad2.

because we'd like to try just living with whatever the
default case-class toString method gives us, rather than having
to maintain a bunch of custom toString methods like we traditionally
did
keep AstVisitor separate from the core AstNode types

nicer than having the `accept` methods polluting our nice
algebraic data types
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