feat: Add double-quoted string constants with escapes

Resolves #20.

syntax/fostr.sdf3

@@ -39,6 +39,7 @@ context-free syntax
 
   Ex.Int       = INT
   Ex.LitString = STRING_LITERAL
+  Ex.EscString = STRING
   Ex.Stream    = <stream>
   Ex.Sum       = <<Ex> +  <Ex>> {left}
   Ex.Gets      = [[Ex] << [Ex]] {left}

tests/hw2.fos

@@ -0,0 +1 @@
+<<< "Hello,\t\tworld!\n\n"

trans/haskell.str

@@ -39,6 +39,7 @@ rules
   hs: (_, Stream())     -> "StdIO"
   hs: (_, Int(x))       -> x
   hs: (_, LitString(x)) -> <haskLitString>x
+  hs: (_, EscString(x)) -> x
   hs: (_, Sum(x, y))    -> $[([x] + [y])]
 
   hs: (Gets(_, xn), Gets(s, x)) -> v

trans/javascript.str

@@ -17,6 +17,7 @@ rules
   js: Stream()     -> $[Stdio]
   js: Int(x)       -> x
   js: LitString(x) -> <javaLitString>x
+  js: EscString(x) -> x
   js: Sum(x,y)     -> $[[x] + [y]]
   js: Gets(x, y)   -> $[[x].gets([y])]
   js: To(x, y)     -> $[to([x],[y])]

trans/python.str

@@ -19,6 +19,7 @@ rules
   py: Stream()     -> $[Stdio]
   py: Int(x)       -> x
   py: LitString(x) -> $[r[x]]
+  py: EscString(x) -> x
   py: Sum(x,y)     -> $[[x] + [y]]
   py: Gets(x, y)   -> $[[x].gets([y])]
   py: To(x, y)     -> $[to([x],[y])]

trans/statics.stx

@@ -184,16 +184,19 @@ constructor was trivial:
 
 Now typing literals is straightforward:
 ```statix
+{! "\git docs/statix_works:trans/statics.stx" extract:
+  start: '(.*ty_Ex.Int.*\s*)'
+  stop: '/. ../'
+!}
+```
   **/
 
-  /** md */
   ty_Ex(Int(_))       = INT().
   ty_Ex(LitString(_)) = STRING().
+  ty_Ex(EscString(_)) = STRING().
   ty_Ex(e@Stream())   = STREAM().
-  /* **/
 
   /** md
-```
 
 Finally we get to the binary operators, and here we use the pattern found in
 recent versions of the