jet +mate arm (#512)

These changes introduce a jet for the [`+mate`
arm](https://developers.urbit.org/reference/hoon/stdlib/2a#mate), and
were tested with the following commands:

```
> (mate ~ ~)
~
> (mate ~ `1)
[~ u=1]
> (mate `2 ~)
[~ u=2]
> (mate `3 `3)
[~ u=3]
> (mate `4 `5)
(error)
took ms/2.118
> ~>  %bout  (roll (gulf 1 1.000) |=([n=@ a=(unit @)] (mate (mate `n ~) (mate ~ `n))))
[~ 1.000]
> =hoon-mate |*([a=(unit) b=(unit)] ?~(b a ?~(a b ?.(=(u.a u.b) ~>(%mean.'mate' !!) a))))
took ms/4.603
> ~>  %bout  (roll (gulf 1 1.000) |=([n=@ a=(unit @)] (hoon-mate (hoon-mate `n ~) (hoon-mate ~ `n))))
[~ 1.000]
```

The `+mate` error text (`bail: fail` followed by stack trace) looks
different from the `+need` error text (`dojo: hoon expression failed`)
despite using the same jet error routine (i.e. `u3m_bail(c3__fail)`).
Feedback on this would be appreciated!

pkg/noun/jets/b/mate.c

@@ -0,0 +1,30 @@
+/// @file
+
+#include "jets/q.h"
+#include "jets/w.h"
+
+#include "noun.h"
+
+
+  u3_noun
+  u3qb_mate(u3_noun a,
+            u3_noun b)
+  {
+    if ( u3_nul == b ) {
+      return u3k(a);
+    } else if ( u3_nul == a ) {
+      return u3k(b);
+    } else if ( c3y == u3r_sing(u3t(a), u3t(b)) ) {
+      return u3k(a);
+    } else {
+      return u3m_error("mate");
+    }
+  }
+  u3_noun
+  u3wb_mate(u3_noun cor)
+  {
+    u3_noun a, b;
+    u3x_mean(cor, u3x_sam_2, &a, u3x_sam_3, &b, 0);
+    return u3qb_mate(a, b);
+  }
+

pkg/noun/jets/q.h

@@ -32,6 +32,7 @@
     u3_noun u3qb_lien(u3_noun, u3_noun);
     u3_noun u3qb_murn(u3_noun, u3_noun);
     u3_noun u3qb_need(u3_noun);
+    u3_noun u3qb_mate(u3_noun, u3_noun);
     u3_noun u3qb_reap(u3_atom, u3_noun);
     u3_noun u3qb_reel(u3_noun, u3_noun);
     u3_noun u3qb_roll(u3_noun, u3_noun);
@@ -254,3 +255,4 @@
     void u3qf_test(const c3_c*, u3_noun);
 
 #endif /* ifndef U3_JETS_Q_H */
+

pkg/noun/jets/tree.c

@@ -2276,6 +2276,8 @@ static u3j_core _139_two__in_d[] =
     {}
   };
 
+static u3j_harm _139_two_mate_a[] = {{".2", u3wb_mate, c3y}, {}};
+
 static u3j_core _139_two_d[] =
 { { "tri", 3, 0, _139_tri_d, no_hashes, _140_tri_ho },
 
@@ -2286,6 +2288,7 @@ static u3j_core _139_two_d[] =
   { "lien", 7, _140_two_lien_a, 0, no_hashes },
   { "murn", 7, _140_two_murn_a, 0, no_hashes },
   { "need", 7, _140_two_need_a, 0, no_hashes },
+  { "mate", 7, _139_two_mate_a, 0, no_hashes },
   { "reap", 7, _140_two_reap_a, 0, no_hashes },
   { "reel", 7, _140_two_reel_a, 0, no_hashes },
   { "roll", 7, _140_two_roll_a, 0, no_hashes },
@@ -2448,3 +2451,4 @@ u3j_Dash = {
   0,
   0
 };
+

pkg/noun/jets/w.h

@@ -32,6 +32,7 @@
     u3_noun u3wb_lien(u3_noun);
     u3_noun u3wb_murn(u3_noun);
     u3_noun u3wb_need(u3_noun);
+    u3_noun u3wb_mate(u3_noun);
     u3_noun u3wb_reap(u3_noun);
     u3_noun u3wb_reel(u3_noun);
     u3_noun u3wb_roll(u3_noun);
@@ -332,3 +333,4 @@
     u3_noun u3wfu_rest(u3_noun);
 
 #endif /* ifndef U3_JETS_W_H */
+