provide access to atoms before conversion to types in reader

[obriencj]

The read function at parse-time will produce a single expression read from the source stream. This could be a cons pair of nested expressions, or just one atomic value. However, the value will have already been through conversion from str to atomic type (int, boolean, float, fraction). Provide a way to read an unconverted atomic value from the stream.

Maybe read-atom to say "give me a thing, but don't take the conversion step." And to supplement that, we can have a process-atom to take that next step.