Checkify: add way to disable categories of errors.

[LenaMartens]

By default only user_asserts are lifted into the checked function.

Current error categories can be freely combined through set operations:

ErrorCategory = enum.Enum('ErrorCategory', ['NAN', 'OOB', 'DIV', 'ASSERT'])

float_errors = {ErrorCategory.NAN, ErrorCategory.DIV}
index_errors = {ErrorCategory.OOB}
automatic_errors = float_errors | index_errors
user_asserts = {ErrorCategory.ASSERT}

[mattjj]

Looks awesome!