Be more explicit with the curve in arithmetic operations

[tuxxy]

What's the issue?

1. When performing arithmetic operations on curve dependent variables (e.g. Point.__mul__), we assume that the curve is the whatever the first variable is.
2. OpenSSL already checks that the inputs are valid for the given curve when performing curve arithmetic, but we should be much more explicit.

How to resolve?

1. Check the curve (via its curve_nid) for each variable in the arithmetic before performing the operation. If the curves do not match, do not perform the operation and raise a TypeError (or other more related error).