Drop IE8 support and update code accordingly

[dmajda]

IE8 is obsolete and its support can be dropped (see the Twitter poll). This allows us to modernize the code a bit.

Tasks:

• Replace arrays.map & friends with native methods
• Replace objects.keys & friends with native methods
• Replace ... instanceof Array with Array.isArray(...)
• Replace suitable for loops with Array methods
• Remove undefined redefinition protection
• Remove various IE8 hacks

[dmajda]

This is done now.

Note some (if not all) features mentioned in the description and used in the code now are available only in IE standards mode, not quirks mode. This means PEG.js does not support IE quirks mode from now on.