add support for ES6 Map's

[nahuel]

E.g.:

const m = new Map([['a', 'b'], ['c', 'd']]); 
// Map { 'a' => 'b', 'c' => 'd' }
const m2 = set(m, _ => _.get('c') )('X')
// Map { 'a' => 'b', 'c' => 'X' }