Changelog for backprop-0.0.3.0

Changelog

Removed samples as registered executables in the cabal file, to reduce dependences to a bare minimum. For convenience, build script now also compiles the samples into the local directory if stack is installed.
Added experimental (unsafe) combinators for working with GADTs with existential types, withGADT, to Numeric.Backprop module.
Fixed broken links in Changelog.

Added optimized numeric Ops, and re-write Num/Fractional/Floating instances in terms of them.
Removed all traces of Summer/Unity from the library, eliminating a whole swath of "explicit-Summer"/"explicit-Unity" versions of functions. As a consequence, the library now only works with Num instances. The API, however, is now much more simple.
Benchmark suite added for MNIST example.

Initial pre-release, as a request for comments. API is in a usable form and everything is fully documented, but there are definitely some things left to be done. (See README.md)