deepmap

Maps with multiple keys per value

Supports the following Map methods:

* clear
* delete
* has
* get
* set

All other Map methods & properties, including well-known symbols and coercion logic, are not catered for at present.

Signature for all methods is the same as for Map, except that in place of a key, it instead accepts an array of keys.

``javascript
map method
`

...DeepMaps instead provide:

`javascript
map method
`

Ergo:

`javascript
const sums = new DeepMap()

sums.set( [ 1, 2 ], 3 )

sums.has( 1 ) // false

sums.has( [ 1 ] ) // false

sums.has( [ 1, 2 ] ) // true

sums.get( [ 1, 2 ] ) // 3

sums.delete( [ 1, 2 ] ) // true

sums.has( [ 1, 2 ] ) // false
``