Support Partial Indexes

[ethanfrey]

Related to #163 (modifying same code logic, good to work on them one after another)

Currently in storage plus, indexes always return a byte key. One common pattern from SQL systems, to save access, is to only index a part of the data.

For example, if we wish to always know the first and last height of all open proposals as an O(1) query, we could create and index (status, height), and assuming #163, then prefix scan over Status::Open to find the first/last height. However, this requires maintaining an index for all the other statuses, we do not care about.

However, and index like INDEX height WHERE status = Status::Open (to borrow from SQL syntax) would only index currently open proposals and record their height, which is exactly what we need and no more.

Design:

1. Indexing function is currently defined as: index: fn(&T) -> Vec<u8> (or with Support composite keys on secondary indexes (multi-index) #163 index: fn(&T) -> K)
   Define partial indexer as index: fn(&T) -> Option<Vec<u8>> (or with Support composite keys on secondary indexes (multi-index) #163 index: fn(&T) -> Option<K>)

2. Create two constructors
   The first one new can work as is, and assume a complete index.
   Add a second constructor that takes a partial indexing function.
   Internally, you can either wrap the complete indexer as a partial indexer that always returns Some(x), or store the two as enum variants.

3. Update save and remove and test them

[ethanfrey]

Blocked: waiting for a clear use case or three to implement. Not just something I think "may be useful someday"

[ethanfrey]

Haven't yet come across a real use case. Closing unless this becomes important in a real project