Add literal string constants (#19)

In addition, for the sake of Haskell code generation, this PR adds static typing
  with Statix.

  Resolves #5.

Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: #19
Co-Authored-By: Glen Whitney <glen@nobody@nowhere.net>
Co-Committed-By: Glen Whitney <glen@nobody@nowhere.net>

syntax/fostr.sdf3

@@ -8,6 +8,14 @@ context-free start-symbols
   
   Start
 
+lexical sorts
+
+  STRING_LITERAL
+
+lexical syntax
+
+  STRING_LITERAL = "'"~[\']*"'"
+
 context-free sorts
 
   Start LineSeq Line OptTermEx TermExLst TermEx Ex
@@ -29,13 +37,14 @@ context-free syntax
 
   TermEx.Terminate = <<Ex>;>
 
-  Ex.Int      = INT
-  Ex.Stream   = <stream>
-  Ex.Sum      = [[Ex] +  [Ex]] {left}
-  Ex.Gets     = [[Ex] << [Ex]] {left}
-  Ex.To       = [[Ex] >> [Ex]] {left}
+  Ex.Int       = INT
+  Ex.LitString = STRING_LITERAL
+  Ex.Stream    = <stream>
+  Ex.Sum       = <<Ex> +  <Ex>> {left}
+  Ex.Gets      = [[Ex] << [Ex]] {left}
+  Ex.To        = [[Ex] >> [Ex]] {left}
 
-  Ex          = <(<Ex>)> {bracket}
+  Ex           = <(<Ex>)> {bracket}
 
 context-free priorities