Reduce identity value

[hal8174]

The reduce function should use an identity value to initialize the accumulator.
Currently, the default constructor is used as the identity. This makes it difficult to change the identity for standard types.
To change the identity of a standard type, you would need to create a wrapping struct.

Options:

1. The identity value is set by a field in the algorithm struct.
2. The identity value is passed as an argument
3. Don't use an identity value and require the reducing function to be commutative.

In my opinion, option 3 is the architecturally prettiest, but it adds uncheckable requirements on the user implemented reducing function.
Option 2 doesn't really work, because the current calling structure prevents the addition of arguments.
Option 1 would be the easy to implement. In addition, this way, the existence of an identity could be checked via concepts.