Can help reduce case-lambda* / lambda* at Tree-IL optimization-time.
* module/language/tree-il/demux-lambda.scm: New file.
* am/bootstrap.am (SOURCES): Add new file.
* module/language/tree-il/optimize.scm (make-optimizer):
* module/system/base/optimize.scm (available-optimizations): Enable
demux-lambda at level 2.
