Update language reference.

Add a glossary and explain the overall shape of a Cretonne function.

cranelift/docs/example.c

@@ -0,0 +1,8 @@
+float
+average(const float *array, size_t count)
+{
+    double sum = 0;
+    for (size_t i = 0; i < count; i++)
+        sum += array[i];
+    return sum / count;
+}