Change 'items' to 'entries'. Deprecate 'items'.
Do not throw errors

It is now clear that FRB should not throw errors if it encounteres
invalid input.  It must propagate null instead.  This is because FRB
inputs do not necessarily change atomically.  The result is that state
must be made consistent by the end of a turn (not enforced), but may
pass through invalid states internally.  As such, throwing an exception
would interfere.
Change asArray to toArray

Since FRB now delegates to JavaScript methods when possible, it behooves
FRB to be consistent with the naming of existing JavaScript methods,
like toString.  asArray would be more correct on the left hand side of a
binding, but that ship has sailed.
Fix toString operator

Previously, FRB was delegating to the `toString` method because the
`toString` operator was not an enumerable key of the operators exports
(non-enumerable inhereited from `Object.prototype`).  This patch
explicitly creates the appropriate observer compiler.
Round out toString, toArray, toMap, toSet

Also alter the toString semantics.  This should not delegate to the
underlying JavaScript method because that will make it more difficult to
replicate in an interoperability layer.  As such, it propagates null for
unrecognized types.
Implement toMap from arrays of entries

Fixes #5

And incidentally get rid of the unnecessary `traverseAll` set in the
`expand` module since we fall through to that case for arbitrary
operators.
Update the change log
0.2.9