add remove to library

For completeness it would be nice to have a remove method for library. Removing from values is easy (values are intended to be unique). The issue is removing from indexes and collections.

Index

Remove from index should:

• remove value from key
• downgrade StashArray to value if length == 1
• remove empty keys

Collection

Remove from collection should:

• remove value from key
• downgrade StashArray to value if length == 1
• replace empty keys with nil
• trim array of trailing nils

In both cases I suppose the idea is to recalculate the key based on the value. This becomes problematic when values map to alternate values...

In collections, an additional issue arises for order. Which value maps when? You could determine this from the index of the values removed.

A slower alternative would be to remove the value and recalculate all indexes and collections. This approach is solid at the expense of speed.