Switch from pretty-simple to pretty-show for native output.

Update tests.

Reason:  it turns out that the native output generated by
pretty-simple isn't always readable by the native reader.
According to https://github.com/cdepillabout/pretty-simple/issues/99
it is not a design goal of the library that the rendered values
be readable using 'read'.  This makes it unsuitable for our
purposes.

pretty-show is a bit slower and it uses 4-space indents
(non-configurable), but it doesn't have this serious drawback.

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