(#21376) Sequence BlockExpressions without nesting

When merging Puppet::Resource::Type instances the code attribute was merged in
a way that caused a deep tree of BlockExpressions. This could cause stack
overflows when evaluating this tree, since evaluation is recursive. This
changes the merge method so that they are sequenced as a flat array.
Merge pull request #1717 from pcarlisle/ticket/21376-stack-overflow

(#21376) Sequence BlockExpressions without nesting