Update libdatatrie to 6ef4485474890606946ed208ff453231b581cdf1

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This PR

• Updates libdatatrie to current master.
• Provides test to illustrate Memory corruption with slighthly bigger alphabets. #74

Resolves #74

[c4f3a0ce]

So it seems that it doesn't resolve the problem completely. It is less prominent, but some segfaults still occur (https://travis-ci.org/pytries/datrie/jobs/622241476).

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Also this falsifying example:

trie = datrie.Trie([chr(i) for i in range(1500)])
trie['ÿ0'] = True
trie['foo'] =  True

'foo' in trie
# True
'ÿ0' in trie                                                                                   
# False

is troubling. Seems related to nil in the alphabet:

trie_without_nil = datrie.Trie([chr(i) for i in range(1, 1500)])
trie_without_nil['ÿ0'] = True

'ÿ0' in trie_without_nil
# True

Which kind of makes sense.

Maybe @thep can shed some light on this?