Added support for new C++ numeric literals. Note: The TeX side of this is rather
jank. Currently, binary literals are formatted just like hexadecimal literals
except with a superscript “b” instead of “#”, and the `'` digit separator
becomes a thin space (`\,` in TeX). If you run `CTANGLE` with the option `+k`,
it will omit the separator in output, so that you can use it in C code as well
as in C++ code.

Added support for C and C++ attributes.

Allowed “`default;`” as a function body.

Added more reserved words: `alignas`, `_Alignas`, `alignof`, `_Alignof`,
`_Atomic`, `_Bool`, `char8_t`, `char16_t`, `char32_t`, `co_await`, `_Complex`,
`consteval`, `constexpr`, `constinit`, `co_return`, `co_yield`, `_Decimal128`,
`_Decimal32`, `_Decimal64`, `decltype`, `_Generic`, `_Imaginary`, `noexcept`,
`_Noreturn`, `nullptr`, `static_assert`, `_Static_assert`, `thread_local`,
`_Thread_local`.

Made `typeid` act like `sizeof`.

Added support for `enum class`, `enum struct`.

Altered handling of `template` and `typename`.  If you supply the option `+t`,
the identifier `N` in

```C++
template<typename N> class C { … };
```

will be made a reserved word so that it acts like a type name.

Quite a few new productions have been added to `prod.w`; they are all at the
end, even though many of them would belong more logically earlier in the file.
There are new kinds of scrap as well, mostly for handling attributes.

To test the major additions, run `CWEAVE` on `testthings.w`.