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/Math.bux

@@ -0,0 +1,44 @@
+module Std::Math {
+
+extern func bux_sqrt(x: float64) -> float64;
+extern func bux_pow(x: float64, y: float64) -> float64;
+extern func bux_abs_i64(x: int64) -> int64;
+extern func bux_abs_f64(x: float64) -> float64;
+extern func bux_min_i64(a: int64, b: int64) -> int64;
+extern func bux_max_i64(a: int64, b: int64) -> int64;
+extern func bux_min_f64(a: float64, b: float64) -> float64;
+extern func bux_max_f64(a: float64, b: float64) -> float64;
+
+func Sqrt(x: float64) -> float64 {
+    return bux_sqrt(x);
+}
+
+func Pow(x: float64, y: float64) -> float64 {
+    return bux_pow(x, y);
+}
+
+func Abs(n: int64) -> int64 {
+    return bux_abs_i64(n);
+}
+
+func AbsF(f: float64) -> float64 {
+    return bux_abs_f64(f);
+}
+
+func Min(a: int64, b: int64) -> int64 {
+    return bux_min_i64(a, b);
+}
+
+func Max(a: int64, b: int64) -> int64 {
+    return bux_max_i64(a, b);
+}
+
+func MinF(a: float64, b: float64) -> float64 {
+    return bux_min_f64(a, b);
+}
+
+func MaxF(a: float64, b: float64) -> float64 {
+    return bux_max_f64(a, b);
+}
+
+}