Custom lexers can now terminate operators

New constraint: custom lexer output is dropped whenever it is used to terminate an operator nested inside another custom lexer, because the recursive call has to return exactly one lexeme

examples/hello-world/main.orc

@@ -1,7 +1,5 @@
-let user = string::concat "lorinc" (string::concat " " "bethlenfalvy")
-let number = 1 + 2
-let interpolated = "Hello $user $number"
+let foo = 1 + 2
+let ffmain = "hello $foo"
 
-let user = r[ "foo" 1, "bar" 3 ]
-let ffmain = user . "foo" |> option::expect "missing value"
-let main = 1
+let user = r[ "foo" 1, "bar" t[3, 4] ]
+let main = user."bar".1