(maint) Make [] operator flatten keys for all LHS types except hash.

It is expected in the Puppet Language that it is possible to write:

$x = ['a', 'b']
SomeType[$x]
(maint) Fix failing test for access operator since array is now ok
(PUP-910) Convert args from 4x representation to 3x

This is a partial solution that is ok as long as there are no functions
written that wants to use the 4x representation.

To be changed in 4.0.0 where functions should be able to opt in.
(maint) Make 3x conversion use verbatim types for non Host/Resource.
(maint) Make Resource::Type use the CodeMerger strategy

This makes the Type.merge method use the CodeMerger instead of
relying on that code knows how to merge itself with any other kind
of code. The CodeMerger is parser specific and is obtained from the
ParserFactory. This way the choice of parser is hidden from the
Type.merge logic.

This also changes the CodeMerger.concatenate to only be responsible
for the actual concatenation (it used to also create a HostClass called
'' as it was used in only one specific use case).
Merge pull request #2167 from hlindberg/maint_future-evaluator-acceptance-test-2

(maint) Make more acceptance tests work for future evaluator