added 'coverage' target to Makefile using luacov

To get more accurate results, we'll likely need to tweak the settings a
bit in .luacov, but at least we now have a basic working setting for
generating coverage reports.

Not yet adding this to `make ci` - we can always do so once we play with
it more , and use a more readable coverage reporter such as
luacov-console, luacov-cobertura, luacov-coveralls, etc.

.luacov

@@ -0,0 +1,8 @@
+-- setting default behaviors for luacov. For documentation on the options,
+-- see https://keplerproject.github.io/luacov/doc/modules/luacov.defaults.html
+
+return {
+  runreport = true,
+}
+
+-- vim: ft=lua

Makefile

@@ -55,7 +55,14 @@ lua-5.3.5/src/liblua-mingw.a: lua-5.3.5
 ci: luacheck testall count
 
 clean:
-	rm -f fennel fennel-bin *_binary.c fennel-bin.exe
+	rm -f fennel fennel-bin *_binary.c fennel-bin.exe built-ins luacov.*
 	make -C lua-5.3.5 clean || true # this dir might not exist
 
-.PHONY: test testall luacheck count ci clean
+coverage: fennel
+	# need a symlink for the fake 'built-ins' module set on macros in fennel.lua
+	ln -s fennel.lua built-ins && rm -f luacov.*
+	$(LUA) -lluacov test/init.lua && rm -f built-ins
+	@echo "generated luacov.report.out"
+	@echo "Note: 'built-ins' coverage is inaccurate because it isn't a real file."
+
+.PHONY: test testall luacheck count ci clean coverage