Forms implementation for %HTML.Trusted. Some backend changes:

* Added Charsets and Character attribute types
* Fix a heavily recursive form of ContentSets, this allows a content-set
  to include another content-set which includes another content-set, and
  so forth.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

TODO

@@ -22,7 +22,6 @@ FUTURE VERSIONS
 
 3.2 release [It's All About Trust] (floating)
  # Implement untrusted, dangerous elements/attributes
-    - Forms are especially wanted
  # Implement IDREF support (harder than it seems, since you cannot have
    IDREFs to non-existent IDs)
  # Frameset XHTML 1.0 and HTML 4.01 doctypes
@@ -44,7 +43,6 @@ FUTURE VERSIONS
     contents should be dropped or not (currently, there's code that could do
     something like this if it didn't drop the inner text too.)
  - Remove <span> tags that don't do anything (no attributes)
- - Remove empty inline tags<i></i>
  - Append something to duplicate IDs so they're still usable (impl. note: the
    dupe detector would also need to detect the suffix as well)
  - Externalize inline CSS to promote clean HTML, proposed by Sander Tekelenburg
@@ -60,8 +58,6 @@ FUTURE VERSIONS
    Also, enable disabling of directionality
 
 5.0 release [To XML and Beyond]
- - AllowedAttributes and ForbiddenAttributes step on the toes of XML by
-   using periods; this needs to be changed.
  - Extended HTML capabilities based on namespacing and tag transforms (COMPLEX)
     - Hooks for adding custom processors to custom namespaced tags and
       attributes, offer default implementation