Worker improvement

1.  Use a single web worker for all deflating/inflating tasks in a ZipReader/ZipWriter, instead of one worker for one entry. This significantly reduces the overhead of creating web workers when processing zip file with many entries. However, users now must call `close()` when they done with ZipReader/ZipWriter, or the worker is leaked.

2.  Separate codecs (Inflater/Deflater) and worker codes, and allow pluggable DEFLATE implementations. Now zip.js can work with zlib-asm and pako.

3. Avoid copy ArrayBuffer's between workers and main thread.

4. Move crc32 into worker, further reduce main thread's load.

No, readability (though using a local variable has a slightly better performance than accessing a variable from the prototype chain). This table is a static constant, by storing it on the prototype, you're suggesting that the variable might me specific to a specific instance of Cr32. By assigning it to a local variable (*outside* Cr32, e.g. at line 100), you clearly show that the table is an private and immutable constant.