Move new-layer insert functionality into its own function.
Create twigs when creating new layers.

Consider inserting these two keys in sequence:

   www.fart/barf/hello/one/two/three
   www.fart/barf/hello/one/two/four

We previously moved through the following states:

   www.fart #0: /barf/hello/one/two/three

   www.fart #0: [unused suffix] LAYER
     /barf/he #0: llo/one/two/three

   www.fart #0: [unused suffix] LAYER
     /barf/he #0: [unused suffix] LAYER
       llo/one/ #0: two/three

   www.fart #0: [unused suffix] LAYER
     /barf/he #0: [unused suffix] LAYER
       llo/one/ #0: [unused suffix] LAYER
         two/thre #0: e

   www.fart #0: [unused suffix] LAYER
     /barf/he #0: [unused suffix] LAYER
       llo/one/ #0: [unused suffix] LAYER
         two/thre #0: e
         two/four #1

That created, initialized, and then rendered unused two suffix
structures for the intermediate layers. But there's no need to
create such structures. We can tell while creating a layer whether
we'll need to split it. Now we do this:

   www.fart #0: /barf/hello/one/two/three

   www.fart #0: [unused suffix] LAYER
     /barf/he #0: LAYER
       llo/one/ #0: LAYER
         two/thre #0: e
         two/four #1

The intermediate layers are created without suffixes. This allocates
about 1/2 as much memory for key suffixes on the URL test.