A few invariants involving subtree-width and black-height balance
could break if singleton nodes were reused; bugs were due to stale
data in the nodes.
The token tree implements the implicit interface in the original
splay-based token tree module in syntax-color/token-tree. However, it
does not appear to perform significantly differently in the case of
indentation yet. It needs some additional profiling and analysis to
see if we can take advantage of the richer structure in the rb tree.
