This gives better coverage and consistency with the scripting APIs, by
whitelisting the primary search scripting API classes and using them instead
of only Map and List methods.

For example, accessing fields can now be done with `.value` instead of `.0`
because `getValue()` is whitelisted. For now, access to a document's fields in
this way (loads) are fast-pathed in the code, to avoid dynamic overhead.

Access to geo fields and geo distance functions is now supported.

TODO: date support (e.g. whitelist ReadableDateTime methods as a start)
TODO: improve docs (like expressions and groovy have for document's fields)
TODO: remove fast-path hack

Closes elastic#18169

Squashed commit of the following:

commit ec9f24b
Author: Robert Muir <rmuir@apache.org>
Date:   Thu May 5 17:59:37 2016 -0400

    cutover to <Def> instead of <Object> here

commit 9edb155
Author: Robert Muir <rmuir@apache.org>
Date:   Thu May 5 17:03:02 2016 -0400

    add fast-path for docvalues field loads

commit f8e38c0
Author: Robert Muir <rmuir@apache.org>
Date:   Thu May 5 16:47:31 2016 -0400

    Painless: add fielddata accessors (.value/.values/.distance()/etc)