Optimize regexp memory usage

[atuchin-m]

Resolves #203
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 new 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 and wrap under a new feature.
In fact, the browser uses adblock from a single sequence => we don't need to synchronize threads, the thread-safety feature should be disabled in browser to get benefit from the optimization.

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

[atuchin-m]

@antonok-edm I'm not sure about enabling the feature in native/Cargo.toml.
Looks like it's used only in node-js and multi-threading doesn't need here.

[antonok-edm]

I pushed a few minor fixes, including:

• 4 space indentation as per standard Rust formatting
• removed manual default() implementation on RegexStorage
• related the feature to object-pooling as an opt-out optimization
• added static assertions for Send and Sync implementations on Engine

This should be all set now, I will merge it and create a new release

[antonok-edm]

Published as v0.5.2 on crates.io