Improved UglifyJS<->SpiderMonkey AST conversions.

* Added directives recognition in SM AST.
* Moved semi-standard SM `Property` type to separate handler.
* Added `const` recognition from SM AST.
* Removed redundant `this`-as-identifier recognition.
* Removed redundant rules for abstract SM types.
* Described `CatchClause` using string syntax.
* Added support for semi-standard `range` tuple as location source.
* Added back-conversion support (to be improved).
Fixes and improvements for UglifyJS->SM AST conversion.

* Explicitly forbidden multiple catch clauses as SM-specific feature.
* Simplified describing of UglifyJS->Mozilla AST conversion rules.
* Moved alias rules to single place.
* Removed usage of dynamic type bindings in generated code (speed-up).
Small fixes for AST conversion.