Basic vector code, without a coordinate system class

All calculations essentially occur in the same frame.
i, j, k and x, y, z are imported as base vectors and base scalars by default on importing sympy.vector.
delop is a Del operator instance.

Example:

>>> v = 3*i - j
>>> delop ^ v
0

(delop ^ v represents curl of v)
Another point in favour of using standard SymPy Core constructs, is that SymPy's core is being rewritten in C++. The C++ implementations of `__eq__` and `__hash__` are likely to be much faster than any Python implementation.