Simplify namespace design.

[AshleyYakeley]

Like this:

• All names are fullnames
• There's a current namespace (initially root) and a list of search namespaces (initially the list containing root)
• Names are looked up in the search namespaces
• Bindings are added to the current namespace
• "namespace" (with a single name) changes the current namespace for its contents, as well as adding a new search namespace
• "as" (with a single name) puts pattern bindings in that namespace (relative to the current namespace)
• "using" allows absolute and relative namespaces and adds a new search namespace

[AshleyYakeley]

This turns out to cause problems, as do all the other designs I considered.

The current design seems to be the least worst.