Optimizers with vector functions

[samuelcohen1]

This PR includes:

• New versions of pullback and pushforward that use plain functions instead of matrices, making them applicable to vectors of vectors. These have completely replaced the matrix versions throughout the code, including for the scalars-only case.
• A test in Optimizers.jl which uses vector functions
• Minor syntax improvements on the Open{Optimizer} constructor to make it more user-friendly

[jpfairbanks]

@samuelcohen1 @tylerhanks should we merge this?