Things can now have slashes in their names

This turns out to be a surprisingly easy tweak, and will allow us to
have hierarchical Tags.

Important note, though: all the operations that were previously
/:thingId/operation have changed their paths!  If we have cheats lurking
around anywhere, they may need fixing.

Refactored RequestHeaderParser out of RequestContext, mostly by
accident, but decided to leave it -- it's a useful featurelet.
Tags are beginning to work

Checkpoint, but a big one:

Introduced TagSetType, as a variant of NameType.

Introduced the notion (although nothing's using it yet) that a PType may
imply a Collection.

Added the _showUnknownName property on Spaces, which is predefined on
System.  This is the page to display for the unknown name.

Added the _tagRefs method, which returns all of the Things that have
tags referring to the received context.

Added LinkFromThingBuilder, which is often useful in QList.from().

Introduced rudimentary rendering support for editing Tag Sets.

Major enhancement to Application.withSpace, to allow situation-specific
error handlers that do smarter things than simply pop back to index.  If
thing() gets a "name not found" error, it invokes a variant of the page
to deal.

To support that, finally enhanced the ThingFailed to carry more
information, so it can be properly processed.  Many small mechanical
changes throughout the code to support this properly.
You can now "edit" a TagThing, which creates it

It intentionally prefills the properties so that if you just say Done,
the resulting reified page is identical to the pseudo-Thing you had with
the Tag.

Created TagThing, a pseudo-Thing that lets us refactor the Tag code to
be more consistent with the rest of the world.
Added the _tagsForProperty method

Along the way, fixed things so you can edit Properties.
Added the very useful _self method
You can now easily see the tags for each Prop

Added the _propsOfType method, which lets you list all of the Tag Set
properties in the Space.  Added that to the default display for a Space.

Made _sort deal with Tag Sets.

Introduced the notion that Properties can declare a special rendering
when you look at them, and more importantly that a *type* can say how to
display properties of that type.  Tag Set properties now default to
listing all of the values of that property.