Add a has-key() function

[ecerulm]

The has-key(obj, 'x') should be equivalent to

is-object($o) and contains("key", $o)

On a first glance it may seems that just .x checks for the existence of key x but

• what if . is an array
• what if . contains key x but the value is null or a falsy value

[larsga]

Hmmmm. It might be useful, but on the other hand it's so easy to do yourself. I'm inclined to wait on this one.