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

bootstrap/source_location.nim

@@ -0,0 +1,8 @@
+type
+  SourceLocation* = object
+    line*: uint32      ## 1-based
+    column*: uint32    ## 1-based (UTF-8 byte offset in line)
+    offset*: uint32    ## byte offset from start of file
+
+proc `$`*(loc: SourceLocation): string =
+  $loc.line & ":" & $loc.column