parser: allow enums to be used as bitfield flags

vlib/compiler/tests/enum_bitfield_test.v

@@ -0,0 +1,34 @@
+[flag]
+enum BfPermission {
+	read
+	write
+	execute
+	other
+}
+
+struct BfFile {
+mut:
+	perm BfPermission
+}
+
+fn test_enum_bitfield() {
+	mut a := BfFile{}
+	a.perm.set(.read)
+	a.perm.set(.write)
+	a.perm.toggle(.execute)
+	a.perm.clear(.write)
+	//a.perm.set(.other)
+
+	assert a.perm.has(.read)
+	assert a.perm.has(.execute)
+	assert !a.perm.has(.write)
+	assert !a.perm.has(.other)
+
+	mut b := BfPermission.read // TODO:  this does nothing currenty just sets the type
+	b.set(.write)
+	b.set(.other)
+	assert b.has(.write)
+	assert b.has(.other)
+	assert !b.has(.read)
+	assert !b.has(.execute)
+}