Prefix tree for mentities name

Because multiple mentities can share the same name, we use a Trie of arrays of mentities.

As for now, we do not index class and property definitions.

TODO add an option.

Property definitions

nitc $ ModelIndex :: _name_prefixes
	# Prefix tree for mentities `name`
	#
	# Because multiple mentities can share the same `name`, we use a Trie of
	# arrays of mentities.
	#
	# As for now, we do not index class and property definitions.
	# TODO add an option.
	var name_prefixes = new Trie[Array[MEntity]]
src/model/model_index.nit:253,2--260,45