Rename BufferManager::Malloc to Memory
Add iterator to Buffer for accessing all elements.

This makes getting to all items in a Buffer more straightforward and
simplifies deserialization.
Consolidated and simplified Item iterators
Fix osmium_serget

This is strange. Before this fix osmium_serget compiled, but the
compiler should have complained, because parse_item() is called with
an argument of type size_t but is defined as:
void parse_item(const Osmium::Ser::TypedItem& item)