Eliminate PersistentVector's implementations usage from default.clj

[si14]

This is a long-term goal, but I think generally it's better to do this.

For now we reuse PersistentVector's implementations in many places in default.clj, coercing a value to persistent vector, performing an operation and coercing it back. This causes slowness (like in 57146955) and bugs (like in #51). We can avoid this in 2 ways:

1. use NDArray instead of vectors, as it is presumably faster;
2. implement defaults using mandatory protocols on arguments.

This options does not contradict with each other, so we can use them both.

[mikera]

Agree in general, NDArray should be able to replace persistent vector usage. In particular, it actually allows the mutable operations to be implemented properly :-)

There might be a few cases where persistent vectors are more efficient - I think we can use them on a case-by-case basis however.