Lambdas rotted because there wasn't any test for them, and after
return type moved to the end, lambdas stayed assumning it in the
front. We fix this and add test program
sextest resolves #+ and #- itself -- it reads the program and prints
what survives to sexc -- so a flag spelling it does not recognise
decides which branch gets compiled
'#+' and '#-' introduce conditional compilation: the form that follows
is kept only when the feature expression is true, and otherwise is read
and thrown away. An expression is a feature name, or and / or / not
of them.
They are read time, not compile time.
Default features are the host's software-version, software-type and
machine-type as CHICKEN reports them, plus what --features flag adds.
