Refactor AST and Parser into modular subdirectories

- Split ast.h into granular headers in v0/ast/
- Split parser.c into modular implementation files in v0/parser/
- Move and rename parser tests to v0/parser/test_*.c
- Update build system (include.mk) with modular sub-makefiles
- Maintain v0/ast.h and v0/parser.h as umbrella headers

v0/ast/module.h

@@ -0,0 +1,34 @@
+#ifndef AST_MODULE_H
+#define AST_MODULE_H
+
+#include "declaration.h"
+#include <stddef.h>
+
+/**
+ * The top-level model.
+ * Every file matches an entire Module.
+ */
+typedef struct {
+    /** @brief The name of the module. */
+    char* name;
+
+    /** @brief The list of imports in the module. */
+    ImportDeclaration* imports;
+
+    /** @brief The number of imports in the module. */
+    size_t import_count;
+
+    /** @brief The list of aliases in the module. */
+    AliasDeclaration* aliases;
+
+    /** @brief The number of aliases in the module. */
+    size_t alias_count;
+
+    /** @brief The list of variables in the module. */
+    VariableDeclaration* variables;
+
+    /** @brief The number of variables in the module. */
+    size_t variable_count;
+} Module;
+
+#endif