make Tree properties hidden and non-enumerable; test it; tweak grunt

Cover was hiding line numbers in stack traces, so I switched back to
non-cover testing by default, and used src instead of dist for tests.

Tree properties get frozen before returning, so the only thing to do
was to declare them 'hidden' on Tree construction.
make Node properties hidden/private; test it; bug fix Tree inflation

Tree inflation had leftover code from before hidden properties.

Node properties are made non-{enumerable|writable|configurable}, and
tests were copied wholesale from Tree to ensure it.
removed defensive code; errors will happen on their own