Use a specific `Literal` to decide how to apply `wrap_kvs` trans functions.

[thorwhalen]

Proposal: Use FirstArgIsMapping(Literal)

A Literal class to mark a function as being one where the first argument is
a mapping (store). This is intended to be used in wrappers such as wrap_kvs
to indicate when the first argument of a transformer function trans like
key_of_id, preset, etc. is the store itself, therefore should be applied as
trans(store, ...) instead of trans(...).

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