feat: add <<< and >>> abbreviating operations to default stream

Resolves #16.

trans/desugar.str

@@ -15,7 +15,9 @@ rules
 
   seqFlatten: Sequence(l) -> Sequence(<mapconcat(?Sequence(<id>) <+ ![<id>])>l)
   
+  defStream:  DefGets(x)  -> Gets(Stream(), x)
+  defStream:  DefTo(x)    -> To(x, Stream())
+
 strategies
 
-  desugar-fostr = bottomup(try(deISe <+ seqFlatten))
- 
+  desugar-fostr = bottomup(try(defStream <+ deISe <+ seqFlatten))

trans/haskell.str

@@ -19,7 +19,8 @@ rules
      returning the final value.
    */
 
-  hs: (_, TopLevel(val)) -> $[import System.IO
+  hs: (_, TopLevel(val)) -> $[-- Preamble from fostr
+  import System.IO
   data IOStream = StdIO
 
   gets :: Show b => a -> b -> IO a

trans/javascript.str

@@ -2,13 +2,16 @@ module javascript
 imports libstrategolib signatures/- util
 
 rules
-  js: TopLevel(x) -> $[const Stdio = {
+  js: TopLevel(x) -> $[// Fostr preamble
+  const Stdio = {
     gets: v => { process.stdout.write(String(v)); return Stdio; },
   }
   function to(data, strm) {
     strm.gets(data);
     return data;
   }
+  // End of preamble
+
   [x]]
 
   js: Stream()     -> $[Stdio]

trans/python.str

@@ -2,7 +2,8 @@ module python
 imports libstrategolib signatures/- util
 rules
 
-  py: TopLevel(x) -> $[import sys
+  py: TopLevel(x) -> $[## Fostr preamble
+  import sys
   class StdioC:
     def gets(self, v):
       print(v, file=sys.stdout, end='')
@@ -11,6 +12,8 @@ rules
     strm.gets(data)
     return data
   Stdio = StdioC()
+  ## End of preamble
+
   [x]]
 
   py: Stream()     -> $[Stdio]