v0.3.0: restructure directories

- src/        ← compiler/selfhost/  (canonical Bux compiler)
- bootstrap/  ← compiler/bootstrap/ (Nim bootstrap)
- lib/        ← library/std/        (standard library)
- rt/         ← library/runtime/    (C runtime)
- tests/      ← compiler/tests/     (unit tests)
- Remove _selfhost/ (built into build/selfhost/ now)
- Update all path references (Makefile, cli.nim, cli.bux, docs)
- Bump version to 0.3.0

lib/Time.bux

@@ -0,0 +1,19 @@
+module Std::Time {
+
+extern func bux_time_ms() -> int64;
+extern func bux_time_us() -> int64;
+extern func bux_sleep_ms(ms: int64);
+
+func Time_NowMs() -> int64 {
+    return bux_time_ms();
+}
+
+func Time_NowUs() -> int64 {
+    return bux_time_us();
+}
+
+func Time_SleepMs(ms: int64) {
+    bux_sleep_ms(ms);
+}
+
+}