[plt-scheme] Efficient asynchronous communication primitives [PATCH]

Per Dimitris:

Note that you need to regenerate cstartup.inc (with make startup, after
temporariy disabling USE_COMPILED_STARTUP in schminc.h) and mzmark.c (with
mkmark.ss) in order to compile it. The latter is of particular importance,
since it is the code that handles traversal of the new fields in the
thread.