improve foramtting of _oasis

* align things better
* get rid of tabs
oasis digest update
added some sexp printing to explain failure
Merge pull request #159 from rgrinberg/master

Fix oasis formatting + add a couple of printers to tests
fixes failing test

return Done instead of Chunk "" when nothing is read
Merge pull request #160 from rgrinberg/fix_tests

fixes failing test
Merge pull request #158 from dsheets/bin_cohttp_server

Improve bin/cohttp_server_*