Implement __repr__ instead of __str__ for python Value

Implement __repr__ instead of __str__ for python Value

Implement the __repr__ method for the various Value classes
as it provides a fallback for __str__ and provides both a
non-ambiguous and human-readable string representation of those
objects.

The Array and Map values' implementation is made more compact by
using list comprehensions, as is done for the Fields API.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>