avoid allocation in Validator.All.contramap

[fwbrasil]

Problem

Tapir uses a default Validatator.All instance with an empty validator sequence and calls contramap on each request handling as part of the value decoding (the call comes from Schema.map)

Solution

Cast the current instance and return it instead of allocating a new one.

Notes

• This is part of Reduce allocation rate #3552
• The cast seems safe since the class is immutable and the empty sequence should have the same behavior regardless of the type parameter.

[adamw]

Thanks!