When checking the result of an ebis_insert() call in an ebtree with unique keys,
if already present, in place of freeing() the old one and return the new one,
rather the correct way is to free the new one, and return the old one. For
this, the __dict_insert() function was folded into dict_insert() as this
significantly simplifies the test of duplicates.
Thanks to Olivier for having reported this bug which came with this one:
"MINOR: dict: Add dictionary new data structure".
When allocating new dictionary entries we store the length of the strings.
May be useful so that not to have to call strlen() too much often at runing
time.
This patch adds minimalistic definitions to implement dictionary new data structure
which is an ebtree of ebpt_node structs with strings as keys. Note that this has nothing
to see with real dictionary data structure (maps of keys in association with values).