More robust numeric expansion

[xaviershay]

The current implementation of numeric expansion (see numericRangeRegex and associated code) doesn't handle some newer additions to range spec such as a1a1..a1a3 and a2a2a1..a2a2. It may be possible to patch up the regex to work, but I suspect we may need to rewrite it with less regex.

In case it's helpful, see https://github.com/xaviershay/hrange/blob/8ef7ed289ac503b6745c324090f09cea1c7cae62/src/Hrange/Parser.hs#L168

These specs are currently marked as pending in expand_test.go.