pipeSync should throw an error on any asynchronous input

[vitaly-t]

When pipeSync is called explicitly by the client, and passed an asynchronous iterable, it should throw an error:

TypeError(`A synchronous iterable was expected: ${JSON.stringify(input)`)

...because this represents an implementation error.

Presently, the input type is only controlled at compile time, and the function erroneously produces an asynchronous output.

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@RebeccaStevens I'm assigning this to yourself, if you want to have more fun with this library ;)