Handle lambdas in lists

[adambaratz]

The added unit test verifies my recent changes to libmustache. The changes to mustache_data_from_array_zval were needed for a less than clear reason. It seemed that when array indices were set explicitly, it didn't end up having any contents visible to the Data destructor, so the associated memory wouldn't be freed. I might want to go back at some point and get rid of the length attribute of Data. It looks like it's mainly used for initialization, so only serves to be something that has to be kept in sync.

[jbboehr]

Looks good!

WRT freeing memory, in handlebars, I'm using a pool allocator. I had looked into using one for mustache, or even using emalloc, but it's a bit more complex for C++. Besides preventing leaks, the other advantage is you don't have to traverse the AST or data trees on destruct.