Rework SexpMaker to reduce overhead.

Changes include:

* Use NodeType instead of node.getClass().getName() processing.
* Cache simple name of node types rather than recalculating.
* Eliminate noop elements from processing.
* Make everything static and avoid SexpMaker alloc.
* Use literal char instead of single-char string in sb.append.
* Preallocate StringBuilder at 100 chars to kickstart growth.

Performance of 100 sexp constructions of the entire AST of 1.8's
parse_rb.rb went from 0.52s to 0.27s. Since I benched it from
Ruby, most of the remaining overhead was JI string conversion, so
I believe the improvement in sexp construction is closer to a 75%
reduction in overhead.

Further improvement for sexp + sha1 calculation would be to feed
the sexp construction directly into sha1 digester rather than
aggregating in an intermediate StringBuilder + String.