Use .b file extension

Co-authored-by: Ezequiel Bellver <ebellver@itba.edu.ar>
Co-authored-by: Juan Barmasch <jbarmasch@itba.edu.ar>

doc/examples/use-case-13.b

@@ -0,0 +1,3 @@
+if ("Hola" + g == 2) {
+    print("Funciona");
+}

doc/examples/use-case-13.txt

@@ -1,3 +0,0 @@
-if ("Hola" + g == 2) {
-	print("Funciona");
-}

doc/examples/use-case-15.b

@@ -0,0 +1,4 @@
+if (1 == 1) {
+    print("Hola");
+}
+}

doc/examples/use-case-15.txt

@@ -1,4 +0,0 @@
-if (1 == 1) {
-	print("Hola");
-}
-}

doc/examples/use-case-8.b

@@ -1,2 +1,3 @@
 fun f = f(3) * x * 4 + 3;
 fun g =  f(3) + f'(3)*(x-3) + f''(3)/2 * (x-3)^2 + f'''(3) / 6 * (x-3)^3 + f''''(3) / 24 * (x-3)^4;
+fun g =  f(2) + integrate(derivative())