trie fixture members are now more explicitly named.
button reader test now uses trie fixture.
Circular buffer updates.

- Added front, back.
- Accessors all now return const ref.
- Updated unit tests.
Defined const iterator for circular buffer. Debating approach.
Simplfied indexing in circular buffer.
Implemented basic const_iterator for circular buffer.
Added binary operator - for circular buffer iterators.
Simplfied stub_c::read using new circular buffer iterators.
Swapped indexing for new front and back methods.
Added clear() to circular_buffer, unit tests.
Fix in trie.

- Search now stops when no matching child node is found. Previously,
search would spin on the last node until the seq ended.
- Added unit test for extra long buffer that exhibited a bug when
searches didn't terminate.
Added debug trie_c::visit.

This is temp. We'll need a better solution in the future if we're
really going to visit the trie. This was just to debug something.
It's coming out, but I wanted it in revision control.
Bashed debug trie_c::visit.
Button reader, unit tests.