Test and refactor of array methods

Changes include:

- e2e tests for array methods.
- Make `run_examples.sh` tell you the difference on fail.
- Unit tests for the `ArrayMethod` struct.
- Small refactor to reduce code duplication.
- Small refactor on `graphemes` to avoid cloning the structure.

examples/array_methods.ion

+echo @split("onetwoone", "two")
+echo @split_at("onetwoone", "3")
+echo @graphemes("onetwo", "3")
+echo @bytes("onetwo")
+echo @chars("onetwo")

examples/array_methods.out

+one one
+one twoone
+o n e t w o
+111 110 101 116 119 111
+o n e t w o

examples/run_examples.sh

@@ -32,7 +32,7 @@ function check_return_value {
     $PROJECT_DIR/target/debug/ion $1 1> $EXAMPLES_DIR/tmp.out 2> /dev/null
 
     # Compare real and expected output
-    cmp --silent $EXAMPLES_DIR/tmp.out $EXPECTED_OUTPUT_FILE
+    diff "$EXAMPLES_DIR"/tmp.out "$EXPECTED_OUTPUT_FILE" > "$EXAMPLES_DIR"/diff_tmp
     local RET=$?
 
     # Clean up the mess
@@ -40,9 +40,12 @@ function check_return_value {
 
     # Write result
     if [[ $RET -ne 0 ]]; then
+        cat "$EXAMPLES_DIR"/diff_tmp
+        rm "$EXAMPLES_DIR"/diff_tmp
         echo -e "Test ${1} ${TAGFAIL}";
         return 1;
     else
+        rm "$EXAMPLES_DIR"/diff_tmp
         echo -e "Test ${1} ${TAGPASS}";
         return 0;
     fi