Fix C++ compiler warnings

[soutaro]

claude-code fixed the problems:

1. include/rbs/parser.h: Renamed field rbs_lexer_t *rbs_lexer_t to rbs_lexer_t *lexer to avoid using the type name as a field name (invalid in C++).
2. src/parser.c:
   - Updated all references from ->rbs_lexer_t to ->lexer
   - Updated struct initializer from .rbs_lexer_t to .lexer
   - Renamed local variables named namespace to ns or out_ns (since namespace is a C++ keyword)
   - Fixed the INTERN_TOKEN macro by removing the unnecessary (void *) cast
   - Added explicit (TypeNameKind) casts for bitwise OR operations on enum values
3. src/location.c: Added explicit cast for rbs_allocator_malloc_impl return value
4. src/util/rbs_allocator.c: Added explicit casts for malloc return values to rbs_allocator_page_t * and rbs_allocator_t *
5. src/util/rbs_constant_pool.c: Added explicit casts for void pointer assignments to rbs_constant_pool_bucket_t * and rbs_constant_t *
6. ext/rbs_extension/legacy_location.c: Added explicit casts for malloc, realloc, and rb_check_typeddata return values
7. ext/rbs_extension/class_constants.c: Removed duplicate declaration of RBS_Parser