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/Path.bux

@@ -0,0 +1,19 @@
+module Std::Path {
+
+extern func bux_path_join(a: String, b: String) -> String;
+extern func bux_path_parent(path: String) -> String;
+extern func bux_path_ext(path: String) -> String;
+
+func Path_Join(a: String, b: String) -> String {
+    return bux_path_join(a, b);
+}
+
+func Path_Parent(path: String) -> String {
+    return bux_path_parent(path);
+}
+
+func Path_Ext(path: String) -> String {
+    return bux_path_ext(path);
+}
+
+}