read-time feature expressions

'#+' 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.

reader.module.scm

@@ -1,3 +1,6 @@
 (module reader (read-from-file
-                read-raw-forms)
+                read-raw-forms
+
+                current-features
+                platform-features)
   "reader.scm")