ecs: add very early-stages entity component system

ecs/README.md

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+# mach/ecs, an Entity Component System for Zig <a href="https://hexops.com"><img align="right" alt="Hexops logo" src="https://raw.githubusercontent.com/hexops/media/master/readme.svg"></img></a>
+
+`mach/ecs` is an Entity Component System for Zig built from first-principles.
+
+## Design principles:
+
+* Clean-room implementation (author has not read any other ECS implementation code.)
+* Solve the problems ECS solves, in a way that is natural to Zig and leverages Zig comptime.
+* Avoid patent infringement upon Unity ECS patent claims.
+* Fast. Optimal for CPU caches, multi-threaded, leverage comptime as much as is reasonable.
+* Simple. Small API footprint, should be natural and fun - not like you're writing boilerplate.
+* Enable other libraries to provide tracing, editors, visualizers, profilers, etc.
+
+## ⚠️ in-development ⚠️
+
+Under heavy development, not ready for use!
+
+As development continues, we're publishing a blog series ["Let's build an Entity Component System from scatch"](https://devlog.hexops.com/categories/lets-build-an-ecs).
+
+Join us in developing it, give us advice, etc. [on Matrix chat](https://matrix.to/#/#ecs:matrix.org) or [follow updates on Twitter](https://twitter.com/machengine).
+
+## Known issues
+
+There are plenty of known issues, and things that just aren't implemented yet. And certainly many unknown issues, too.
+
+* Missing multi-threading!
+* Currently only handles entity management, no world management or scheduling. No global data, etc.
+* Lack of API documentation (see "example" test)
+* Missing hooks that would enable visualizing memory usage, # of entities, components, etc. and otherwise enable integration of editors/visualizers/profilers/etc.
+* TypedEntities does not allow for storage of sparse data yet, and also specifically comptime defined sparse data - both are needed. e.g. if only a handful of entities need a component but there are hundreds of thousands.
+* If many entities are deleted, TypedEntities iteration becomes slower due to needing to skip over entities in the free_slots set, we should add a .compact() method that allows for remediating this as well as exposing .
+* If *tons* of entities are deleted, even with .compact(), memory would not be free'd / returned to the OS by the underlying MultiArrayList. We could add a .compactAndFree() method to correct this.
+* It would be nicer if there were configuration options for performing .compactAndFree() automatically, e.g. if the number of free entity slots is particularly high or something.
+* Currently we do not expose an API for pre-allocating entities (i.e. allocating capacity up front) but that's very important for perf and memory usage in the real world.
+* When TypedEntities is deinit'd, entity Archetypes - or maybe systems via an event/callback, need a way to be notified of destruction.
+* Entities.get() currently operates on the archetype type name, but we should perhaps enable also getting a TypedEntities instance via passing a unique string name. e.g. if you want to store two separate team's Player entities in distinct TypedEntities collections.
+* There should exist a Merge/Combine function for composing Archetypes from components and other Archetypes without explicitly listing every single component out in a new struct.
+
+## Copyright & patent mitigation
+
+The initial implementation was a clean-room implementation by Stephen Gutekanst without having
+read other ECS implementations' code, but with speaking to people familiar with other ECS
+implementations. Contributions past the initial implementation may be made by individuals in
+non-clean-room settings (familiar with open source implementations only.)
+
+Critically, this entity component system stores components for a classified archetype using both
+a multi array list (independent arrays allocated per component) as well as hashmaps for sparse
+component data for optimization. This is a novel and fundamentally different process than what
+is described Unity Software Inc's patent US 10,599,560. This is not legal advice.

ecs/build.zig

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+const std = @import("std");
+
+pub fn build(b: *std.build.Builder) void {
+    const mode = b.standardReleaseOptions();
+    const target = b.standardTargetOptions(.{});
+
+    const lib = b.addStaticLibrary("ecs", "src/main.zig");
+    lib.setBuildMode(mode);
+    lib.setTarget(target);
+    lib.install();
+
+    const main_tests = b.addTest("src/main.zig");
+    main_tests.setBuildMode(mode);
+
+    const test_step = b.step("test", "Run library tests");
+    test_step.dependOn(&main_tests.step);
+}