v0.3.0

• Add an erase_prefix method.
• Add a shrink_to_fit method.
• Fix the pair of iterators returned by equal_range, the second iterator was always equal to the first.
• The src/ directory was renamed tsl/ for coherence with other projects.
• Fix the iterator erase(const_iterator first, const_iterator last) method. The invalidation of the last iterator was not taken into account.
• Fix issue #6 where erase may leave a trie_node without a value_node nor any child. It can cause problems on iteration.