A vector of patterns which generally make up a set of patterns for which to check
usefulness against.
All patterns in the matrix must be of equal length
Struct representing if a pattern is useful nor not. witnesses are a list of
patterns which are not matched. Not guaranteed to list all patterns, but is guaranteed
to list at least one witness if the pattern is useful
A witness of the usefulness of a pattern. During construction, patterns
are pushed from left to right, with constructors containing fields “consuming”
fields already on the stack for its parameters
Checks if pattern is useful with respect to prev_patterns, that is, if the
pattern matches some value which is not matched by any pattern in prev_patterns