Add Bool special form to handle type expressions in boolean contexts (#31)

- Add `_LiteralGeneric` as the resulting type of `IsSub` and other `<type-bool>`. This handles execution in boolean contexts.
- Add `Matches[Lhs, Rhs]` which is equivalent to `IsSub[Lhs,Rhs] and IsSub[Rhs,Lhs]`
- Add `Bool` which converts `Literal` to `_LiteralGeneric` (eg. `Bool[Literal[False]]`
- Add `AllOf`, which checks that all elements of a `<variadic-type-arg> +` are one of `Literal[True]` or `_LiteralGeneric[True]`.
- Add `AnyOf`, which checks that any element of a `<variadic-type-arg> +` is one of `Literal[True]` or `_LiteralGeneric[True]`.

Author: dnwpark