Unify String representation of Maps

[simonvonhackewitz]

I've found different implementations of as_string:

Format	Implementation
(1 => one), (2 => two)	Mutable_Map, ps_map
{2=two, 1=one}	mutable_tree_map
[(1,one), (2,two)]	hash_map

[michaellilltokiwa]

I like this one: {(1 => one), ...}

@simonvonhackewitz Can you find out how other languages do it?

[simonvonhackewitz]

python
{1: 'one', 2: 'two'}

Java
{1=one, 2=two}

Haskell
fromList [(1,"one"),(2,"two")]