shell: rough in code for async i/o
shell: more async output scaffolding.
shell: add locking to the output thread.
shell: make the evaluator asyn as well.
shell: continue with the async output reporting.
shell: The mutex is not actually needed.
shell: cleanup, before going fully async
shell: serialize the scheme evaluator.
shell: more serialization work.
shell: more async output hacking around
shell: fix deadlock from the previous commit
shell: improve documentation
shell: fully async printing in guile
async evaluator: fix double-print
async output: one last, final fix.
evaluators: restore the older backward-compat API
Merge pull request #1017 from linas/async

Async output for scheme shell