The current method, listing codes to disable and a list of exceptions to
that list, still has trouble with some cases. For example, disabling a
standard, re-enabling a category within that standard, then ignoring or
disabling a sniff within that category cannot be handled. We'd need a
list of exceptions to the exceptions, and possibly a list of exceptions
to that list too, and figuring out how to keep those lists up to date as
new directives are encountered could prove to be confusing.

Since the standard→category→sniff→code hierarchy is supposed to be
thought of as a tree, let's store the ignore list that way instead.
Manipulating the branches of the tree is straightforward no matter what
directives are encountered.

In this implementation I've favored speed over space: there are cases
where we could prune a subtree that would evaluate to "ignore" or "don't
ignore" for any possible input, but detecting that doesn't seem worth
the time when it's not likely there will be so many enable or disable
directives that the wasted space will be a problem.

Fixes PHPCSStandards#111

The current method, listing codes to disable and a list of exceptions to
that list, still has trouble with some cases. For example, disabling a
standard, re-enabling a category within that standard, then ignoring or
disabling a sniff within that category cannot be handled. We'd need a
list of exceptions to the exceptions, and possibly a list of exceptions
to that list too, and figuring out how to keep those lists up to date as
new directives are encountered could prove to be confusing.

Since the standard→category→sniff→code hierarchy is supposed to be
thought of as a tree, let's store the ignore list that way instead.
Manipulating the branches of the tree is straightforward no matter what
directives are encountered.

In this implementation I've favored speed over space: there are cases
where we could prune a subtree that would evaluate to "ignore" or "don't
ignore" for any possible input, but detecting that doesn't seem worth
the time when it's not likely there will be so many enable or disable
directives that the wasted space will be a problem.

Fixes PHPCSStandards#111