Follow ups to deprecating ES3 support and native method overrides

[slevithan]

In XRegExp v4.0.0, ES3 support was removed (see #108) and XRegExp.install('natives') was removed (#207).

Some follow ups that should be done as a result:

• Consider deleting the "fixed" versions of native methods and just defer to native regex/string methods. ES5+ browsers don't have most of the old bugs and inconsistencies.
• Bring back most of the tests from s-xregexp-natives.js (deleted in Remove support for XRegExp.install('natives') #207) that test native regex capabilities, and adapt them for XRegExp methods like exec and replace. The replace tests described under heading "provides extensions:" was brought back in commit 6439765.
• Update comments in s-xregexp-methods.js that say things like "These specs should mirror those for RegExp.prototype.exec as closely as possible", since s-xregexp-natives.js was deleted in Remove support for XRegExp.install('natives') #207.
• Possibly kill handling for inconsistencies related to nonparticipating capturing groups (search for "nonparticipating" in code), if ES5 browsers generally get this right.
• Possibly kill the empty character class ([] and [^]) transforms if all major ES5 browsers get this right. (Moved to Remove obsolete cross-browser fix for empty character classes #319)
• Remove calls to stored versions of native regex/string methods, since native methods are no longer overridden.