Polish the curve equation checking functions and tests

[AntoineRondelet]

See: thread #68 (comment)

More specification, as mentioned by @dtebbs in #68 (comment), we need to:

• Tests for the is_well_formed method (generate negative tests cases by just addign 1 to the Y coord, etc)
• Whether to use is_well_formed to check curve points on read? (since it will generally be faster) (remembering that we should also have a flag for this)
• Whether to use a generic equation-checking function in the main body? (in fact some implementations of is_well_formed may be able to reuse it)