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/scope.nim

@@ -0,0 +1,46 @@
+import std/tables
+import types, ast
+
+type
+  SymbolKind* = enum
+    skVar
+    skFunc
+    skType
+    skConst
+    skModule
+
+  Symbol* = ref object
+    kind*: SymbolKind
+    name*: string
+    typ*: Type
+    decl*: Decl        ## optional back-reference to AST decl
+    isMutable*: bool
+    isPublic*: bool
+    isOwn*: bool       ## true if declared as own T
+
+  Scope* = ref object
+    parent*: Scope
+    table*: Table[string, Symbol]  ## O(1) lookup via hash table
+
+proc newScope*(parent: Scope = nil): Scope =
+  result = Scope(parent: parent)
+
+proc define*(scope: Scope, sym: Symbol): bool =
+  ## Returns false if name already exists in this scope
+  if scope.table.hasKey(sym.name):
+    return false
+  scope.table[sym.name] = sym
+  return true
+
+proc lookup*(scope: Scope, name: string): Symbol =
+  var cur = scope
+  while cur != nil:
+    if cur.table.hasKey(name):
+      return cur.table[name]
+    cur = cur.parent
+  return nil
+
+proc lookupLocal*(scope: Scope, name: string): Symbol =
+  if scope.table.hasKey(name):
+    return scope.table[name]
+  return nil