Use an explicit dumper map for the auto format.

Use an explicit dumper map for the auto format.

The dumper chosen in auto format is the last one registered, if both the
text and binary ones are available. For all the builtin types the
preferred format is binary, except for strings and for json.

Dumping strings as text by default was handled by a special case; now
they are just handled by registering the text dumper after the binary
one. Json is now text by default, because it's marginally more
efficient.