Move all the query system to its own package. The reason is to
avoid it to rely on unexported methods and structures, and move
it out of the main package since this is really not a core
feature. It is still tied to the toml.TomlTree and toml.Position
structures for now.
* Move query mechanism to its own subpackage
* Rename QueryResult to Result to avoid stutter
* Add query.CompileAndExecute
Fixes#116
Kazuyoshi Kato