Add function for retrieving current namespace

[khoek]

This is very easy. There are two options, since if we are currently in a namespace the builtin tactic.open_namespaces returns the current namespace as the first entry:

1. Modify the code so that open_namespaces prepends name.anonymous to the list it currently returns when open_namespaces is called outside any namespace, or

2. Just add a new library function to do this.

In keeping with the namespace/monad for tactic.open_namespaces, this function should have type tactic name. Note that tactic (option name) or anything of the link is unnecessary because the semantically correct value when no namespace is open is just name.anonymous.

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