Fix indexed fast paths for exotic objects

Fix indexed fast paths for exotic objects

Previously, Object.values() and Object.entries() erased the concrete type of
an exotic receiver, while indexed property fast paths accepted any object
with the fast_array bit.  Typed arrays could consequently be interpreted as
njs_array_t objects, causing invalid memory reads and writes.

The fix is to preserve the receiver type during enumeration and to use
njs_is_fast_array() for both indexed get and set fast paths.

The issue was introduced in 69072164 (0.8.6); the overbroad fast path dates
to 912ab387 (0.3.8).

This closes #1098 issue on GitHub.