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.
2d77f23f0b