move app instrumentation to instrument.rkt
add observe
check result of observed expression

Checking that an observation actually takes effect
is difficult. Needs more work.
add check-observe
tidy up
docs for observe
observe works with mem
added deflazy, defmem convenience macros
misc minor improvements
fix contract
use observe in linear regression example
some examples from forestdb (scalar implicature, cf CP4)
fix little bugs
added church-compat for examples
add last scalar implicature example