string: added ensure_trailing_slash().
resource: reworked resource_package and resource_manager.

resource_package:
	removed load_manifest(), added open() and close();
	replaced root property with path, added name;
	simple resource "streams" (need a proper slice stream, and
	don't want to load the chunk into memory);
	read new package format (packed only).¹

resource_manager:
	added base_path property;
	reworked for resource_package changes.

1: Might support the unpacked form in the future, but I expect
tooling to provide better dynamic support via networking.
It's better that the game code has less going on; less for us to
deal with.
test/resource/manager: updated to resource_package & resource_manager changes.

test/data/pkg: replaced old unpacked package data with packed data.
test/data/project: added test project for tool_build.¹

1: The two packages are used for test/resource/manager.