Optimize regexp memory usage

[atuchin-m]

Currently, the regexp field consumes a lot of memory because of the size of Arc<RwLock<Option<Arc<CompiledRegex>>>>.
It takes 40 bytes per item even when None is stored.

Most of the rules is non-regex(~90%).
This approach takes 8 bytes per item for such rules.
It saves about 7Mb of memory in total.

Important note: this makes the engine not thread-safe.
In fact, the browser uses adblock from a single sequence => we don't need to synchronize threads.

P.S. Many Arc in the code are left untouched to make the diff reasonable.

Memory usage after loading data/rs-ABPFilterParserData.dat in tests/deserialization.rs
(Release, ubuntu x64, added jemallocator locally to calculate):
before: 42094712 bytes allocated/53563392 bytes resident after: 34332128 bytes allocated/48881664 bytes resident

[antonok-edm]

Thanks! I think this is a reasonable change, but we do need to be aware of other projects that use adblock-rust - could we gate this behind an off-by-default cargo feature, so that everything is still thread-safe by default?

[atuchin-m]

@antonok-edm I've recreated the PR (the current was done from a fork): #204
As for the feature: I can do it if we need to. Let's discuss in slack.