Basic support of asm.js uglification

A had a problem when trying to uglify asm.js modules in my [asmcrypto.js](https://github.com/vibornoff/asmcrypto.js) project.

To fix that issue I added basic support:
* Asm.js is turned on by directive "use asm", so when it is met,
* Name mangler doesn't reuse names in the inner scopes of the modules, and
* Compression is turned off for that modules.