Add compiler benchmarks

[gcampax]

As the ThingTalk compiler grows in complexity, we should benchmark & profile the time spent doing:

• parsing
• typechecking
• compilation to JS code
• compilation of JS code

This way, we can decide whether it is meaningful to have a compiled code cache for frequently used commands, and at what level (JS code or JS function), and whether JSIR-level optimizations (eg dead code elimination, common subexpression elimination) are meaningful, when the JS backend compiler cannot do them easily.