Bugfix: support nil keys

Fix #26: insert multiple nil keys should not panic

Memory and performance optimization release

• Optimize node16/48 space usage
• Optimize node16 performance using bits package instead of sort
• Minor changes and code refactoring

Fix infra code

• Update travis to support go v1.13, v1.14 and v1.15
• Fix go v1.15 vet errors
• Restore test coverage to 100%

Bugfix: tree traversal with conditional callback

Fix #14: The tree traversal did not stop in some cases, even when the callback returned false.