I'm not comfortable with these changes being applied to a mainline branch, 
so I've moved them to [pyk-command-named-colon].

Rationale is that names like : or :foo reduce the utility of [namespace 
which] and break with documented behaviour ("two or more colons" are a 
namespace separator).  Further, as the TclOO segfault below illustrates, 
this is a can of worms likely to uncover implicit assumptions elsewhere 
in the core.

I suspect there's useful stuff in the patch, but I'd like to see a more 
considered approach .. preferably including unifying the name 
canonicalisation code referred to below.