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/Io.bux

@@ -0,0 +1,28 @@
+module Std::Io {
+
+extern func PrintLine(s: String);
+extern func Print(s: String);
+extern func PrintInt(n: int);
+extern func PrintInt64(n: int64);
+extern func PrintFloat(f: float64);
+extern func PrintBool(b: bool);
+extern func ReadLine() -> String;
+extern func bux_read_file(path: String) -> String;
+extern func bux_write_file(path: String, content: String) -> int;
+extern func bux_file_exists(path: String) -> int;
+
+func ReadFile(path: String) -> String {
+    return bux_read_file(path);
+}
+
+func WriteFile(path: String, content: String) -> bool {
+    let r: int = bux_write_file(path, content);
+    return r != 0;
+}
+
+func FileExists(path: String) -> bool {
+    let r: int = bux_file_exists(path);
+    return r != 0;
+}
+
+}