Switch the test to make sure we are measuring the constructor

Since TrieMap.empty() now returns a shared instance, we really want to
measure the constructor.

Signed-off-by: Robert Varga <robert.varga@pantheon.sk>
Merge pull request #17 from rovarga/test-new-instantiation

Switch the test to make sure we are measuring the constructor
Restore non-empty shared instance

Restores the TrieMap.empty() behavior from 0.2.0, plus reuses the
equivalence function.

Signed-off-by: Robert Varga <robert.varga@pantheon.sk>
Merge remote-tracking branch 'upstream/master'