ec.c, catacomb/__init__.py: Return tail from `parse'; provide `fromstring'.

ec.c, catacomb/__init__.py: Return tail from `parse'; provide `fromstring'.

`ECCurve.parse' was inconsistent with the established nomenclature.
Generally, `parse' reads a prefix of some given string, returning the
denoted value and the unprocessed remainder of the input string;
`fromstring' does the same, except that it returns only the value, and
raises an exception if the remainder of the string is not only
whitespace.

Fix `ECCurve.parse' (which unfortunately is an incompatible change), and
add missing `fromstring' methods to `ECCurve' and `ECInfo'.