Delay rule compilation and cache the results

[tstromberg]

This PR moves rule compilation until the last possible moment and caches the result.

This ensures that path typos such as mal scan ./nonexistent return an error instantly instead of compiling the rules first.

Some example performance improvements:

• mal xyz: 40X faster
• mal scan ./nonexistent: 25X faster
• make test: 9X faster

This does not improve the result of mal scan /path/to/malcontent-samples however.

[egibs]

This is awesome. Funnily enough, I was thinking about caching compiled rules yesterday.