Add enum_names() BIF to return names of an enum type's values

testing/btest/Baseline/bifs.enum_names/out

@@ -0,0 +1,13 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+{
+RED,
+GREEN,
+PURPLE,
+BLUE
+}
+{
+RED,
+GREEN,
+PURPLE,
+BLUE
+}

testing/btest/bifs/enum_names.zeek

@@ -0,0 +1,10 @@
+# @TEST-EXEC: zeek -b %INPUT >out
+# @TEST-EXEC: btest-diff out
+
+type Color: enum { RED, GREEN, BLUE };
+type ColorAlias: Color;
+
+redef enum Color += { PURPLE };
+
+print enum_names(Color);
+print enum_names(ColorAlias);