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/Fs.bux

@@ -0,0 +1,19 @@
+module Std::Fs {
+
+extern func bux_dir_exists(path: String) -> int;
+extern func bux_mkdir_if_needed(path: String) -> int;
+extern func bux_list_dir(dir: String, ext: String, out_count: *int) -> *String;
+
+func DirExists(path: String) -> bool {
+    return bux_dir_exists(path) != 0;
+}
+
+func Mkdir(path: String) -> bool {
+    return bux_mkdir_if_needed(path) != 0;
+}
+
+func ListDir(dir: String, ext: String, count: *int) -> *String {
+    return bux_list_dir(dir, ext, count);
+}
+
+}