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convert abt-io to use json-c and follow margo's init_ext() and json format convention

Closes #4

See merge request !4

- unrelated to the purpose of this branch, but necessary to get asan to
  pass while testing changes.  One of these is important and would have
  leaked a small amount of memory with every blocking abt_io call

switch to mochi-cfg (json) configuration

See merge request !2

- also add simple test program that exercises a few basic functions,
  including fallocate().

- problem reported by Marc Vef
- fixes #3