feat: Type-dependent Haskell code generation

Caveat: type is still not being assigned for the Sequence()
  constructor.

  Also fixes the parsing of literal strings (whitespace just after
  the opening quote was being ignored, and was ambiguous just
  before the opening quote).

syntax/fostr.sdf3

@@ -14,7 +14,7 @@ lexical sorts
 
 lexical syntax
 
-  STRING_LITERAL = ~[\']*
+  STRING_LITERAL = "'"~[\']*"'"
 
 context-free sorts
 
@@ -38,7 +38,7 @@ context-free syntax
   TermEx.Terminate = <<Ex>;>
 
   Ex.Int       = INT
-  Ex.LitString = <'<STRING_LITERAL>'>
+  Ex.LitString = STRING_LITERAL
   Ex.Stream    = <stream>
   Ex.Sum       = <<Ex> +  <Ex>> {left}
   Ex.Gets      = [[Ex] << [Ex]] {left}