Minor fixes to StdNonZeroNumberProvider for gdb

While looking at the pretty-printers, I found a few minor oddities in
StdNonZeroNumberProvider.

First, gdb.Type.fields() already returns a sequence, so there's no
need to call list().

Second, it's more idiomatic for the (somewhat misnamed) to_string
method to simply return the underlying gdb.Value.  This also lets gdb
apply whatever formats were passed to `print`, as the new test shows.

Third, there's no need to use the field's name when looking up a field
in a value, the gdb.Field itself can be used.