Add method keys for EnumerableMap

[vlad-khramov]

🧐 Motivation

Add convenient way for retrieving all keys of EnumerableMap. I understand that this method will have limited usage (as EnumerableSet.values())

📝 Details
There are possibilities to get all the keys of EnumerableMap, but they are quite inconvenient. Examles for EnumerableMap.UintToUintMap:

        bytes32[] memory temp = map._inner._keys._inner._values;
        uint256[] memory result;

        assembly {
            result := temp
        }

or

using EnumerableSet for EnumerableSet.Bytes32Set;
...
        bytes32[] memory temp = map._inner._keys.values();
        uint256[] memory result;

        assembly {
            result := temp
        }

or even loop by all keys.

I propose to add method keys with imlementation as in the example 2 above