Don't require NIX_SHOW_STATS for NIX_COUNT_CALLS
Document NIX_SHOW_STATS and NIX_COUNT_CALLS
Add a typeOf primop

We already have some primops for determining the type of a value, such
as isString, but they're incomplete: for instance, there is no isPath.
Rather than adding more isBla functions, the generic typeOf function
returns a string representing the type of the argument (e.g. "int").
Add a test of the type primops
Document typeOf