Add tasty.reflect.utils

[nicolasstucki]

With implementation of def let(rhs: Term)(in: Term.Ident => Term): Term.

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This version allows sealing terms even if the type is not statically known while keeping the type safety guarantees on quoted expressions.

Added reflective tpe on quoted.Expr[T] to get its quoted.Type[T] even when T is just an existential type.

Added asExprOf to cast an Expr[_] to an Expr[U]. Fails if the expression is not a subtype of U.

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Use asExprOf[T] when needed.

[liufengyun]

LGTM, except that the repeated usage of seal.asExprOf suggests that we should keep a short-cut API for friendliness.

[nicolasstucki]

Actually the current definition of tpe is unsound.

[nicolasstucki]

I removed the alternative to #5651 which was unsound. The solution will be an improvement on sealing types that will be needed but we can work around it for now with casts.