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/Process.bux

@@ -0,0 +1,14 @@
+module Std::Process {
+
+extern func bux_process_run(cmd: String) -> int;
+extern func bux_process_output(cmd: String) -> String;
+
+func Process_Run(cmd: String) -> int {
+    return bux_process_run(cmd);
+}
+
+func Process_Output(cmd: String) -> String {
+    return bux_process_output(cmd);
+}
+
+}