libpostal aus unit numbers

[missinglink]

This PR fixes a bug in libpostal when parsing AUS unit numbers, as per pelias/pelias#753

The PR assumes that the order of forward-slash delimited address numerals is in the order {unit}/{house number}. I did some checking online and consulted https://en.wikipedia.org/wiki/Address_(geography) and it seems to be a fairly safe bet.

You can do a ctrl+f on that wiki page and search for slash, the only other countries listed as possibly having a slash are Czech Republic and Slovakia.

In both of those cases, the slash represents an addressing scheme which we don't currently support, so I think that the change will be hugely positive for Australia while having no real negative impact elsewhere.

The PR is in 4 parts:

• refactor to use an index: as more 'bugfixes' are added, we need to avoid iterating the array multiple times, so this introduces an index and also refactors an existing bugfix to take advantage of it.
• parse the Aussie unit numbers
• enable field mapping for 'unit', for some reason we were not mapping 'unit' from libpostal to our schema, I can't see any reason why, so I've enabled it since we are working towards better unit number support.
• additional test cases to cover variants provided by OP

resolves: pelias/pelias#753

[orangejulius]

Sweet!