Support repeatable directives

[rmosolgo]

• Parse them and print them
• Allow them to be repeated in queries

Fixes #3571

[rmosolgo]

Oops, I meant to run benchmarks before merging, but I forgot. So I ran them now:

diff --git a/before.txt b/after.txt
index 2d5e40214..c58b71ecf 100644
--- a/before.txt
+++ b/after.txt
@@ -1,16 +1,16 @@
 Warming up --------------------------------------
-scan - introspection   113.000  i/100ms
+scan - introspection   111.000  i/100ms
 parse - introspection
-                        83.000  i/100ms
-    scan - fragments   194.000  i/100ms
-   parse - fragments   142.000  i/100ms
+                        82.000  i/100ms
+    scan - fragments   192.000  i/100ms
+   parse - fragments   140.000  i/100ms
     scan - big query    18.000  i/100ms
    parse - big query    14.000  i/100ms
 Calculating -------------------------------------
-scan - introspection      1.126k (± 1.9%) i/s -      5.650k in   5.018545s
+scan - introspection      1.100k (± 2.2%) i/s -      5.550k in   5.049252s
 parse - introspection
-                        825.404  (± 2.7%) i/s -      4.150k in   5.031397s
-    scan - fragments      1.930k (± 3.1%) i/s -      9.700k in   5.031685s
-   parse - fragments      1.383k (± 2.5%) i/s -      6.958k in   5.033406s
-    scan - big query    193.983  (± 1.5%) i/s -    972.000  in   5.011623s
-   parse - big query    139.962  (± 3.6%) i/s -    700.000  in   5.008677s
+                        821.984  (± 2.8%) i/s -      4.182k in   5.092036s
+    scan - fragments      1.896k (± 2.2%) i/s -      9.600k in   5.066591s
+   parse - fragments      1.369k (± 2.3%) i/s -      6.860k in   5.012959s
+    scan - big query    189.862  (± 2.1%) i/s -    954.000  in   5.027203s
+   parse - big query    139.087  (± 4.3%) i/s -    700.000  in   5.041601s

I wanted to be sure that the new Racc version wasn't any slower, and it looks good to me.