Allow platforms to implement custom encoders for speed

[GoogleCodeExporter]

We spend a lot of time in encoding HTML and URLs so there is the need to 
optimize this code. For Pharo we have the methods #includesUnsafeUrlCharacter: 
and #includesUnsafeXmlCharacter: on platform which don't really help anybody 
else. GemStone has some patches to Seaside-Core to make it run faster for them, 
Issue 482 outlines some things that could be done to make it go faster on GST.

The idea is to keep the existing HTML and URL encoders as default ones but 
allow the platform to implement optimized ones if they so chose.

Original issue reported on code.google.com by philippe...@gmail.com on 19 Dec 2010 at 4:05